Let tox run lint, mypy, and html envs without 3.9

The tox environments that are not duplicated per Python version were set to run on Python 3.9, for consistency with Cygwin, where 3.9 is the latest available (through official Cygwin packages), so output can be compared between Cygwin and other platforms while troubleshooting problems. However, this prevented them from running altogether on systems where Python 3.9 was not installed. So I've added all the other Python versions the project supports as fallback versions for those tox environments to use, in one of the reasonable precedence orders, while keeping 3.9 as the first choice for the same reasons as before.

Author: EliahKagan

tox.ini

@@ -11,20 +11,20 @@ commands = pytest --color=yes{posargs}
 
 [testenv:lint]
 description = Lint via pre-commit
-base_python = py39
+base_python = py{39,310,311,312,38,37}
 set_env =
 SKIP = black-format
 commands = pre-commit run --all-files --hook-stage manual
 
 [testenv:mypy]
 description = Typecheck with mypy
-base_python = py39
+base_python = py{39,310,311,312,38,37}
 commands = mypy -p git
 ignore_outcome = true
 
 [testenv:html]
 description = Build HTML documentation
-base_python = py39
+base_python = py{39,310,311,312,38,37}
 deps = -r doc/requirements.txt
 allowlist_externals = make
 commands =