tox: use deps = . instead of pip install

If a user is using `tox-uv` (a plugin for tox that supports running the
various `pip` commands vi `uv`), then `pip install .` is seen as an
external command, which would require `allowlist_externals`. Instead,
install the local project by specifying it as a dep. Then, regular tox
and tox-uv both will install it as `pip install .` or `uv pip install
.`, respectively.

tox.ini

@@ -72,8 +72,8 @@ commands =
 
 
 [testenv:check-self]
+deps = .
 commands =
-    pip install -e .
     pydoclint --config=pyproject.toml .