tests: Fix tox error in py36 use virtualenv<20.22.0

tox uses virtualenv under its hood for managing virtual environments.
Virtualenv 20.22.0 dropped support for Python <= 3.6.

Since we want to test against Python 3.6 we need to make sure we use
a version of virtualenv earlier than 20.22.0.

This error was not stopping any tests from passing but was printed
multiple times to stderr when executing the py36 target:

  Error processing line 1 of [...]/.tox/py36/[...]/_virtualenv.pth:

    Traceback (most recent call last):
      File "/usr/lib/python3.6/site.py", line 168, in addpackage
        exec(line)
      File "<string>", line 1, in <module>
      File "[...]/.tox/py36/[...]/_virtualenv.py", line 3
        from __future__ import annotations
                                         ^
    SyntaxError: future feature annotations is not defined

Source: https://tox.wiki/en/latest/faq.html#testing-end-of-life-python-versions
Change-Id: I27bd8200987ecf745108ee8c7561a365f542102a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/387694
Tested-by: Daniel Kutik <daniel.kutik@lavawerk.com>
Commit-Queue: Daniel Kutik <daniel.kutik@lavawerk.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

tox.ini

@@ -16,6 +16,7 @@
 
 [tox]
 envlist = lint, py36, py37, py38, py39, py310, py311
+requires = virtualenv<20.22.0
 
 [gh-actions]
 python =