Porting notes:

- tox handles Python environments automatically. Remove a bit of PYTHONPATH
  manipulation (that was using Python 2 code which always failed!)

- No `clean` target: `git clean -xdf` should fit the bill.

- No `reindent` target: the `reindent.py` script it was using does not
  exist (anymore?).

- No equivalent of tox-test-coverage, which was an artifact of the past,
  using nose.  Instead, the test-coverage target only is ported, which
  uses pytest, and works.

* Add support for Python 3.11

* Test Python 3.12-dev

* Add colour to CI for readability

* Drop support for EOL Python 3.6

* Test slower Windows first to speed up CI

Replace lxml with html.parser for the structural-diff.

This includes a new structural diff which is more robust, as it handles
changes like different attribute order.

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.

First commit to address #1718.

* Add support for Python 3.10

* Drop the dot https://twitter.com/pytestdotorg/status/753767547866972160

Co-authored-by: Matthäus G. Chajdas <Anteru@users.noreply.github.com>

* Add regexlint to tox so linting can be done during development

Tested on Windows.

Closes #1556

* Make regex linting depend on pygments/regexlint@master