Clean up more around base64.decodebytes() calls

[foolip]

Follow-up to #28747.

[foolip]

Care must be taken when merging this and #28789. Both change webdriver/tests/support/asserts.py, and the second to land also needs to remove the import six statement. But I don't know which will land first yet.

[foolip]

I'm fairly confident I didn't break the tests here:

• Chrome – no changes
• Firefox – looks like existing flakiness
• Safari – looks like existing flakiness

[foolip]

Thanks @jgraham! Just to be sure I've rebased to trigger another run, and will check if any of the changes are consistent regressions before landing.

[foolip]

Alright, in the latest run Firefox had no regressions. Safari did, and here's the combination of 4 runs:
https://wpt.fyi/results/webdriver/tests?q=seq%28status%3Apass%20status%3A%21pass%20status%3Apass%20status%3A%21pass%29&run_id=5755592649998336&run_id=5760575348932608&run_id=5640824303910912&run_id=5731672098078720

With that, the only candidate regression is "AssertionError: session not created (500): Could not create a session: The Safari instance terminated while trying to pair with it." which I'm confident I didn't cause here.