Upgrade to Pygments 2.3.1

Version 2.3.1
-------------
(released Dec 16, 2018)

- Updated lexers:

  * ASM (PR#784)
  * Chapel (PR#735)
  * Clean (PR#621)
  * CSound (PR#684)
  * Elm (PR#744)
  * Fortran (PR#747)
  * GLSL (PR#740)
  * Haskell (PR#745)
  * Hy (PR#754)
  * Igor Pro (PR#764)
  * PowerShell (PR#705)
  * Python (PR#720, #1299, PR#715)
  * SLexer (PR#680)
  * YAML (PR#762, PR#724)

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

Version 2.3.0
-------------
(released Nov 25, 2018)

- Added lexers:

  * Fennel (PR#783)
  * HLSL (PR#675)

- Updated lexers:

  * Dockerfile (PR#714)

- Minimum Python versions changed to 2.7 and 3.5
- Added support for Python 3.7 generator changes (PR#772)
- Fix incorrect token type in SCSS for single-quote strings (#1322)
- Use `terminal256` formatter if `TERM` contains `256` (PR#666)
- Fix incorrect handling of GitHub style fences in Markdown (PR#741, #1389)
- Fix `%a` not being highlighted in Python3 strings (PR#727)