tests: drop tox support

This hasn't been working out as well as we'd hope.  Tox relies on
the system having Python versions installed which distros don't
tend to carry anymore.  Our custom run_tests leverages vpython
when possible to run stable Python 3.8 & 3.11 versions which is
providing an OK level of coverage in practice.

Change-Id: Ida517f7be47ca95703e43bc0af5a24dd70c0467e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/540001
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>

CONTRIBUTING.md

@@ -43,17 +43,12 @@ probably need to split up your commit to finer grained pieces.
 
 Lint any changes by running:
 ```sh
-$ tox -e lint -- file.py
+$ flake8
 ```
 
 And format with:
 ```sh
-$ tox -e format -- file.py
-```
-
-Or format everything:
-```sh
-$ tox -e format
+$ black file.py
 ```
 
 Repo uses [black](https://black.readthedocs.io/) with line length of 80 as its
@@ -73,15 +68,11 @@ the entire project in the included `.flake8` file.
 [PEP 8]: https://www.python.org/dev/peps/pep-0008/
 [flake8 documentation]: https://flake8.pycqa.org/en/3.1.1/user/ignoring-errors.html#in-line-ignoring-errors
 
+
 ## Running tests
 
-We use [pytest](https://pytest.org/) and [tox](https://tox.readthedocs.io/) for
-running tests.  You should make sure to install those first.
-
-To run the full suite against all supported Python versions, simply execute:
-```sh
-$ tox -p auto
-```
+We use [pytest](https://pytest.org/) for running tests.  You should make sure to
+install that first.
 
 We have [`./run_tests`](./run_tests) which is a simple wrapper around `pytest`:
 ```sh