Support the NO_COLOR environment variable

[akaihola]

Setting NO_COLOR to any non-empty value will disable syntax highlighting even on terminal output. This overrides the color = true setting in pyproject.toml, but will be overridden by PY_COLORS=1 or the --color command line option, if those are present.

Fixes #355.

Previously, #353 added a command line options, a pyproject.toml option and the PY_COLORS environment variable to for enabling/disabling syntax highlighting.

See earlier discussion in #353, and pypa/pip#10909, for the reasoning behind supporting both NO_COLOR and PY_COLORS.

• decide whether it would make sense to support FORCE_COLOR as well (see this comment in pypa/pip)
• add suppert for FORCE_COLOR
• empty value of NO_COLOR and FORCE_COLOR should be considered as activating that option, similar to how Pytest works
• tests
• ensure fast performing tests
• change log
• code review

[akaihola]

@roniemartinez & @MatthijsBurgh, in addition to adding support for the NO_COLOR environment variable, I've now converted the tests for syntax highlighting configuration into parametrized matrix tests as much as possible.

There are now four test functions:

• test_should_use_color_no_pygments() verifies that all combinations of configuration do disable syntax highlighting if pygments is not present (108 cases)
• test_should_use_color_pygments_and_command_line_argument()` checks that for all combinations of other configuration options, the command line arguments dictate use of syntax highlighting (72 cases)
• test_should_use_color_pygments_and_py_colors() makes sure PY_COLORS={0|1} overrides pyproject.toml and NO_COLOR in all of their combinations (24 cases)
• test_should_use_color_pygments() goes through the remaining combinations in which pyproject.toml, NO_COLOR and tty/non-tty determine use of syntax highlighting in absence of other configuration options (12 cases)

To reduce duplication and boilerplate, I've created some parametrized fixtures instead of straight using @pytest.mark.parametrize. This way the patching of environment variables, creating the pyproject.toml file and patching the return value of sys.stdout.isatty are done inside the fixtures instead of separately in each test function.

I still suspect that for someone fresh looking at the tests, the resulting 256 lines of Python code are more difficult to understand and maintain than the 254 lines (here still just 146 lines since NO_COLOR wasn't yet implemented) it would take by enumerating each parameter combination separately.

[roniemartinez]

LGTM