Clean up 'color' environment variables.

The TOX_TESTENV_PASSENV hasn't been useful for some time and by its mere presence wasted a lot of time today under the assumption that it's doing something. Instead, just rely on one variable FORCE_COLOR. If it's not honored, then that should be the fix upstream.

.github/workflows/main.yml

@@ -6,26 +6,8 @@ permissions:
   contents: read
 
 env:
-  # Environment variables to support color support (jaraco/skeleton#66):
-  # Request colored output from CLI tools supporting it. Different tools
-  # interpret the value differently. For some, just being set is sufficient.
-  # For others, it must be a non-zero integer. For yet others, being set
-  # to a non-empty value is sufficient. For tox, it must be one of
-  # <blank>, 0, 1, false, no, off, on, true, yes. The only enabling value
-  # in common is "1".
+  # Environment variable to support color support (jaraco/skeleton#66)
   FORCE_COLOR: 1
-  # MyPy's color enforcement (must be a non-zero number)
-  MYPY_FORCE_COLOR: -42
-  # Recognized by the `py` package, dependency of `pytest` (must be "1")
-  PY_COLORS: 1
-  # Make tox-wrapped tools see color requests
-  TOX_TESTENV_PASSENV: >-
-    FORCE_COLOR
-    MYPY_FORCE_COLOR
-    NO_COLOR
-    PY_COLORS
-    PYTEST_THEME
-    PYTEST_THEME_MODE
 
   # Suppress noisy pip warnings
   PIP_DISABLE_PIP_VERSION_CHECK: 'true'