Log Message:
py-pygments: updated to 2.3.1

Version 2.3.1

- Updated lexers:
  * ASM
  * Chapel
  * Clean
  * CSound
  * Elm
  * Fortran
  * GLSL
  * Haskell
  * Hy
  * Igor Pro
  * PowerShell
  * Python
  * SLexer
  * YAML

- Fix invalid string escape sequences
- Fix FutureWarning introduced by regex changes in Python 3.7

Version 2.3.0

- Added lexers:
  * Fennel
  * HLSL

- Updated lexers:
  * Dockerfile

- Minimum Python versions changed to 2.7 and 3.5
- Added support for Python 3.7 generator changes
- Fix incorrect token type in SCSS for single-quote strings
- Use terminal256 formatter if TERM contains 256
- Fix incorrect handling of GitHub style fences in Markdown
- Fix %a not being highlighted in Python3 strings