Log Message:
Upgrade py-pygal from 1.7.0 to 2.3.1

This upgrade fixes compatibility with new lxml.

Upstream changelog
==================
2.3.1

_This is a micro release and I have very little time on my hands right now sorry_

    Fix crash with no values when the print_values_position param is set (thanks \ 
@cristen)

2.3.0

    New call API: chart = Line(fill=True); chart.add('title', [1, 3, 12]); \ 
chart.render() can now be replaced with Line(fill=True)(1, 3, 12, \ 
title='title').render()
    Drop python 2.6 support

2.2.3

    Fix bar static value positioning (#315)
    Add stroke_opacity style (#321)
    Remove useless js in sparklines. (#312)

2.2.2

    Add classes option.
    Handle ellipsis in list type configs to auto-extend parent. (Viva python3)

2.2.0

    Support interruptions in line charts (thanks @piotrmaslanka #300)
    Fix confidence interval reactiveness (thanks @chartique #296)
    Add horizontal line charts (thanks @chartique #301)
    There is now a formatter config option to format values as specified. The \ 
formatter callable may or may not take chart, serie and index as argument. The \ 
default value formatting is now chart dependent and is value_formatter for most \ 
graph but could be a combination of value_formatter and x_value_formatter for \ 
dual charts.
    The human_readable option has been removed. Now you have to use the \ 
pygal.formatters.human_readable formatter (value_formatter=human_readable \ 
instead of human_readable=True)
    New chart type: SolidGauge (thanks @chartique #295)
    Fix range option for some Charts (#297 #298)
    Fix timezones for DateTimeLine for python 2 (#306, #302)
    Set default uri protocol to https (should fix a lot of "no \ 
tooltips" bugs).

2.1.1

    Import scipy as a last resort in stats.py (should workaround bugs like #294 \ 
if scipy is installed but not used)

2.1.0

    Bar print value positioning with print_values_position. Can be top, center \ 
or bottom (thanks @chartique #291) ci doc
    Confidence intervals (thanks @chartique #292) data doc

2.0.12

    Use custom xml_declaration avoiding conflict with processing instructions

2.0.11

    lxml 3.5 compatibility (#282)

2.0.10

    Fix transposable_node in case all attributes are not there. (thanks @yobuntu).

2.0.9

    Add dynamic_print_values to show print_values on legend hover. (#279)
    Fix unparse_color for python 3.5+ compatibility (thanks @felixonmars, @sjourdois)
    Process major labels as labels. (#263)
    Fix labels rotation > 180 (#257)
    Fix secondary axis
    Don't forget secondary series in table rendering (#260)
    Add defs config option to allow adding gradients and patterns.

2.0.8

    Fix value overwrite in map. (#275)

2.0.7

    Fixing to checks breaking rendering of DateTimeLine and TimeDeltaLine (#264) \ 
(thanks @mmrose)
    Fix render_in_browser. (#266) (#268) (thanks @waixwong)

2.0.6

    Avoid x label formatting when label is a string

2.0.5

    Fix x label formatting

2.0.4

    Fix map coloration

2.0.3

    Fix label adaptation. (#256)
    Fix wrong radar truncation. (#255)

2.0.2

    Fix view box differently to avoid getting a null height on huge numbers. (#254)
    Fix broken font_family default
    Fix non namespaced svg (without embed) javascript by adding uuid in config \ 
object. (config is in window.pygal now).

2.0.1

    Fix the missing title on x_labels with labels.
    Auto cast to str x labels in non dual charts (#178)
    Add print_labels option to print label too. (#197)
    Add value_label_font_family and value_label_font_size style options for \ 
print_labels.
    Default print_zeroes to True
    (Re)Add xlink in desc to show on tooltip
    Activate element on tooltip hovering. (#106)
    Fix radar axis behaviour (#247)
    Add tooltip support in metadata to add a title (#249).
    Take config class options in account too.

2.0.0

    Rework the ghost mechanism to come back to a more object oriented behavior, \ 
storing all state in a state object which is created on every render. (#161)
    Refactor maps
    Add world continents
    Add swiss cantons map (thanks @sergedroz)
    Add inverse_y_axis options to reverse graph (#24)
    Fix DateTimeLine time data loss (#193)
    Fix no data for graphs with only zeroes (#148)
    Support value formatter for pie graphs (#218) (thanks @never-eat-yellow-snow)
    Add new Box plot modes and outliers and set extremes as default (#226 #121 \ 
#149) (thanks @djezar)
    Add secondary_range option to set range for secondary values. (#203)
    Maps are now plugins, they are removed from pygal core and moved to packages \ 
(pygal_maps_world, pygal_maps_fr, pygal_maps_ch, ...) (#225)
    Dot now supports negative values
    Fix dot with log scale (#201)
    Fix y_labels behaviour for lines
    Fix x_labels and y_labels behaviour for xy like
    Improve gauge a bit
    Finally allow call chains on add
    Transform min_scale and max_scale as options
    mode option has been renamed to a less generic name: box_mode
    fix stack_from_top for stacked lines
    Add flake8 test to py.test in tox
    Remove stroke style in style and set it as a global / serie configuration.
    Fix None values in tables
    Fix timezones in DateTimeLine
    Rename in Style foreground_light as foreground_strong
    Rename in Style foreground_dark as foreground_subtle
    Add a render_data_uri method (#237)
    Move font_size config to style
    Add font_family for various elements in style
    Add googlefont:font support for style fonts
    Add tooltip_fancy_mode to revert to old tooltips
    Add auto print_value color + a configurable value_colors list in style
    Add guide_stroke_dasharray and guide_stroke_dasharray in style to customize \ 
guides (#242) (thanks @cbergmiller)
    Refactor label processing in a _compute_x_labels and _compute_y_labels \ 
method. Handle both string and numbers for all charts. Create a Dual base chart \ 
for dual axis charts. (#236)
    Better js integration in maps. Use the normal tooltip.