Log Message:
py-graphviz: updated to 0.15

Version 0.15
------------

``Graph`` and ``Digraph`` instances created via the context-manager returned
by ``subgraph()`` now (re)use ``directory``, ``format``, ``engine``, and
``encoding`` from the parent instead of using defaults (behavioral change).
Note that these attributes are only relevant when rendering the
subgraph independently (i.e. as a stand-alone graph) from within the
``with``-block, which was previously underdocumented. To reflect that
the DOT language does not allow subgraph statements to specify ``strict``
(i.e. no way to override the setting of the containing graph), instances
created via the context-manager are now ``strict=None`` instead of ``False``
(so they continue to render stand-alone as non-strict by default).

Drop Python 3.5 support and tag Python 3.9 support.

Add documentation link to new upstream installation procedure for Windows.