tox: Don't write byte code (maybe)

In tox versions after 3.0.0rc1 [1], setting the environment variable
PYTHONDONTWRITEBYTECODE will cause tox not to write .pyc files, which
means you don't have to delete them, which makes things faster.

In older tox versions, the env var is ignored.

If we bump the minimum tox version to something later than 3.0.0rc1, we
can remove the commands that find and remove .pyc files.

Conflicts:
	tox.ini

NOTE(stephenfin): Conflict is due to a number of unrelated changes made
during Rocky, such as Idda28f153d5054efc885ef2bde0989841df29cd3.

[1] 336f4f6bd8

Change-Id: I779a17afade78997ab084909a9e6a46b0f91f055
(cherry picked from commit 590a2b6bbf)
(cherry picked from commit 99f0c4c014)

tox.ini

@@ -17,6 +17,9 @@ setenv = VIRTUAL_ENV={envdir}
          OS_STDOUT_CAPTURE=1
          OS_STDERR_CAPTURE=1
          OS_TEST_TIMEOUT=160
+         # NOTE(efried): This is only effective in tox versions after 3.0.0rc1
+         # https://github.com/tox-dev/tox/commit/336f4f6bd8b53223f940fc5cfc43b1bbd78d4699
+         PYTHONDONTWRITEBYTECODE=1
 deps = -r{toxinidir}/test-requirements.txt
 commands =
   find . -type f -name "*.pyc" -delete