Replace py2isms with py3

[wchargin]

We have by now firmly and securely dropped Python 2 support. As of
#4361, all our sources set srcs_version = "PY3", so no Bazel-built py2
binaries may depend on these sources (even Google-internally).

There remain many contortions in our code that are vestiges of the
bicompatibility period. Most of these fall under “mostly harmless, but
should be cleaned up incrementally or eventually”.

Off the top of my head:

• Replace u"xxx" literals with just "xxx" strings (Upgrade Black to v20.8b1 and drop py27 and py35 #4409)
• Replace box = [None] closure access pattern with nonlocal (Use nonlocal rather than box[0] in tests #4482)
• Remove now-useless __future__ imports (py3: remove __future__ imports #4503)
• Use os.makedirs(exist_ok=True) rather than EEXIST dances
• Remove fallbacks for sys.stderr.buffer, et al.
• Remove fallbacks for html.escape (py3: use html.escape unconditionally #4515)
• Remove the argparse_util fallback (py3: remove argparse_util helper #4493)
• Simplify conditional imports of unittest.mock (py3: remove mock backport #4507)
• Kill the unittest.mock backport dependency (py3: remove mock backport #4507)
• Replace all usages of six with the idiomatic Python 3 code (pending: py3: excise six #4510)

Feel free to add more.