Have tox skip linting unless requested, for now

This is to make it so simple `tox` usage has the expected property
of leaving all source code files in the working tree unchanged.

Linting how sometimes performs auto-fixes since gitpython-developers#1862, and the
pre-commit command in tox.ini, which had also run `black --check`,
will do even more file editing due to the changes in gitpython-developers#1865.

The bifurcation for black into separate mutating and non-mutating
hooks, introduced in 5d8ddd9 (gitpython-developers#1693), was not carried over into
Ruff autoformatting in gitpython-developers#1865. But also it:

- Was not necessarily a good approach, and likely should not be
  preserved in any form. It was an unusual and unintuitive use of
  pre-commit. (It can be brought back if no better approach is
  found, though.)

- Was done to avoid a situation where it was nontrivial to set up
  necessary dependencies for linting in the GitPython virtual
  environment itself, because flake8 and its various plugins would
  have to be installed.

  They were not listed in any existing or newly introduced extra
  (for example, they were not added to test-requirements.txt) in
  part in the hope that they would all be replaced by Ruff, which
  happened in gitpython-developers#1862.

- Already did not achieve its goal as of gitpython-developers#1862, since it was
  (probably rightly) not extended to Ruff linting to use/omit --fix.

Now that Ruff is being used, people can run `pip install ruff` in a
virtual environment, then run the `ruff` command however they like.
This takes the place of multiple tools and plugins.

The situation with the tox "lint" environment is thus now similar
to that of the tox "html" environment when it was added in e6ec6c8
(gitpython-developers#1667), until it was improved in f094909 (gitpython-developers#1693) to run with
proper isolation.

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 requires = tox>=4
-env_list = py{37,38,39,310,311,312}, lint, mypy, html
+env_list = py{37,38,39,310,311,312}, mypy, html
 
 [testenv]
 description = Run unit tests