GeezerVison

We only need to test pairs of environment variables that set conflicting values.
For example, NO_COLOR=1 and TERM=dumb should be tested with FORCE_COLOR=1 and PYTHON_COLOR=1. Tests for a single environment variable which forces color (FORCE_COLOR and PYTHON_COLOR) should be performed when the fallback value determined by isatty or _supports_virtual_terminal is False.

We need also to test can_colorize() when stderr is a TTY, but stdout is not, and with an explicit file argument. Including corner cases: no file.fileno, no file.isatty, file.isatty() and os.isatty(file.fileno()) return different results. BTW, I think that file.isatty() should perhaps be tested before os.isatty(file.fileno()).

This is a large issue and can be done in a separate PR.

Only the first two checks are necessary. Others do not add anything useful. They look out of place and make the intention of the tests less clear.

The first group tests the different values of the variables. The second group tests relative priority of the variables (it does not repeat tests for different values). Then the same is done for PYTHON_COLORS -- first the different values of PYTHON_COLORS (including the empty value), then its relative priority to other variables.

LGTM! 👍