chore(dependencies): Update pygments to 2.3.1

[pyup-bot]

This PR updates pygments from 2.3.0 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

Links

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