Update pygments to 2.3.1

[pyup-bot]

This PR updates Pygments from 2.2.0 to 2.3.1.

Changelog

2.3.1

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

- Updated lexers:

* ASM (PR784)
* Chapel (PR735)
* Clean (PR621)
* CSound (PR684)
* Elm (PR744)
* Fortran (PR747)
* GLSL (PR740)
* Haskell (PR745)
* Hy (PR754)
* Igor Pro (PR764)
* PowerShell (PR705)
* Python (PR720, 1299, PR715)
* SLexer (PR680)
* YAML (PR762, PR724)

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

2.3.0

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

- Added lexers:

* Fennel (PR783)
* HLSL (PR675)

- Updated lexers:

* Dockerfile (PR714)

- Minimum Python versions changed to 2.7 and 3.5
- Added support for Python 3.7 generator changes (PR772)
- Fix incorrect token type in SCSS for single-quote strings (1322)
- Use `terminal256` formatter if `TERM` contains `256` (PR666)
- Fix incorrect handling of GitHub style fences in Markdown (PR741, 1389)
- Fix `%a` not being highlighted in Python3 strings (PR727)

Links

• PyPI: https://pypi.org/project/pygments
• Changelog: https://pyup.io/changelogs/pygments/
• Homepage: http://pygments.org/

[coveralls]

Coverage remained the same at 86.27% when pulling 8612524 on pyup-update-pygments-2.2.0-to-2.3.1 into e806b35 on master.