automation: run hg with python3
Python 2.7 support will go away soon. Let's use Python 3 as part of
the automation.
Differential Revision: https://phab.mercurial-scm.org/D12260
rust: enable Python 3 support unconditionally
Note: `cpython/python3-sys` is a default feature.
Differential Revision: https://phab.mercurial-scm.org/D12316
cext: really remove Python 2 file handling code
Disclaimer: This is _WIN32 code and I have no machine to test.
cext: backout
e9ca736f5b52 "remove Python 2 file handling code"
It's if"n"def.