Prohibit contrast degradation for styles via test

[not-my-profile]

First PR to address #1718.

Web accessibility is important. Unfortunately currently many pygments
styles have rules with poor contrasts. This commit introduces a test
case that fails if the minimum contrast of a style gets worse, e.g:

E  AssertionError: contrast degradation for style 'borland'
E    The following rules have a contrast lower than the required 2.9:
E
E    * 1.90 Token.Text.Whitespace
E    * 2.80 Token.Generic.Heading
E    * 2.30 Token.Generic.Subheading
E
E  assert not 1.9 < 2.9

This is accomplished by storing the current minimum contrasts in
./tests/contrast/min_contrasts.json.

When you improve a minimum contrast the test fails with:

E  AssertionError: congrats, you improved a contrast! please run ./scripts/update_contrasts.py
E  assert not 1.9 > 0.9

Running the script as instructed updates the JSON file, making the test pass.

New styles are required to meet the WCAG AA contrast minimum of 4.5.

[Anteru]

Thanks, that looks good. I wonder if the WCAG package should be part of the developer dependencies, if look at tox we already pull in pytest and there's no reason why we can't pull in more dependencies there. There's no need to copy/paste it into this project and it's super error prone as well.

[Anteru]

Thanks a lot, merged!