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>

.github/workflows/test-ci.yml

@@ -27,6 +27,6 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        python -m pip install tox tox-gh-actions
-    - name: Test with tox
-      run: tox
+        python -m pip install pytest
+    - name: Run tests
+      run: python -m pytest