Additional updates for compatibility with tox 4

Additional updates for compatibility with tox 4

'[tox] skipsdist' behaves differently in tox 4 [1]. In addition, setting
'[testenv] basepython' with '[tox] ignore_basepython_conflict' has been
the cause of a few tox 4 bugs (most since fixed, thankfully) and might
be deprecated [2]. Remove it since we don't need it in any of our
environments.

[1] https://github.com/tox-dev/tox/issues/2730
[2] https://github.com/tox-dev/tox/issues/2846

Signed-off-by: Stephen Finucane <stephen@that.guru>