Use -m build instead of setup.py.

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.

Improve checks.

* Fix lots of small errors.
* Remove the line length check.
* Add an option to skip lexers with no alias
* Run checks in make check
* Add a new CI target.

The PR #1819 provides a tool to identify unique token types. This PR
aims to remove the most obvious cases of unicorn styles which are used
in a single lexer only.