build(tox): install documentation dependencies from the doc group

The documentation build dependencies (Sphinx and its extensions) used
to be the `doc` extra. 9e54de4 (gitpython-developers#2243) moved them to a PEP 735 `doc`
dependency group in `pyproject.toml`, and CI now installs them with
`pip install --group doc`, but the tox `html` environment still asked
for the `doc` extra. GitPython no longer has that extra, so
`tox -e html` failed before building anything:

    html: failed with extras not found for package GitPython: doc
    (available: gix, test)

Install the `doc` dependency group with `dependency_groups = doc`
instead. `extras =` is set to empty, so the environment still installs
only GitPython and the documentation dependencies, as it did with
`extras = doc`, rather than inheriting `extras = test` from
`[testenv]`.

`dependency_groups` was added in tox 4.22, so raise the `requires`
floor from `tox>=4` to `tox>=4.22`. With an older tox installed, tox
then provisions a new enough one rather than ignoring the setting.

Run locally with tox 4.67.1, `tox -e html` now installs the group and
builds the HTML documentation successfully. Sphinx reports one
warning, about the docstring of
`git.objects.submodule.util.is_relative_url`. CI's documentation build
reports the same single warning.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: EliahKagan

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-requires = tox>=4
+requires = tox>=4.22
 env_list = py{38,39,310,311,312,313,314,315}, ruff, format, mypy, html, misc
 
 [testenv]
@@ -51,7 +51,8 @@ ignore_outcome = true
 [testenv:html]
 description = Build HTML documentation
 base_python = py{39,310,311,312,38}
-extras = doc
+extras =
+dependency_groups = doc
 allowlist_externals = make
 commands =
     make BUILDDIR={env_tmp_dir}/doc/build -C doc clean