Replace py2isms with py3

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 (#4409)
• Replace box = [None] closure access pattern with nonlocal (#4482)
• Remove now-useless __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 (#4515)
• Remove the argparse_util fallback (#4493)
• Simplify conditional imports of unittest.mock (#4507)
• Kill the unittest.mock backport dependency (#4507)
• Replace all usages of six with the idiomatic Python 3 code (pending: #4510)

Feel free to add more.

The issue names several remaining Python 3 cleanup areas but no specific files or tests. Choose one unchecked item, locate its usages in the sources, and confirm that the selected Python 2 compatibility code is removed without changing supported Python 3 behavior.