Add tox environment to build HTML documentation

The main use of this, similar to the step at the end of
pythonpackage.yml, is to find errors produced by building.

However, actual documentation *is* built, and unlike other tox
environments, running this one actually writes outside the .tox/
directory, creating the documentation in the usual target
location. For that reason, this environment is omitted from the
env_list, so that it does not run by default and unexpectedly
overwrite documentation that may recently have been built before
changes are made that could cause generated documentation to be
different.

tox.ini

@@ -11,16 +11,25 @@ commands = pytest --color=yes {posargs}
 
 [testenv:lint]
 description = Lint via pre-commit
-basepython = py39
+base_python = py39
 commands = pre-commit run --all-files
 
 [testenv:mypy]
 description = Typecheck with mypy
-basepython = py39
+base_python = py39
 commands = mypy -p git
 ignore_outcome = true
 
 [testenv:black]
 description = Check style with black
-basepython = py39
+base_python = py39
 commands = black --check --diff git
+
+# Run "tox -e html" for this. It is deliberately excluded from env_list, as
+# unlike the other environments, this one writes outside the .tox/ directory.
+[testenv:html]
+description = Build HTML documentation
+base_python = py39
+deps = -r doc/requirements.txt
+allowlist_externals = make
+commands = make -C doc html