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.

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 =