automation: drop support for Python 2.7 in Windows environment We stop installing Python 2.7 in the Windows environment. We remove support for building Python 2.7 wheels and installers. There is still some Python 2.7 support cleanup to perform in automation. But this removes the biggest remaining chunk of references to 2.7. Differential Revision: https://phab.mercurial-scm.org/D12263

automation: drop support for Python 2.7 in Linux environment We stop installing Python 2.7 via pyenv. We stop installing the system Python 2 packages. We delete support for running tests on Python 2.7. Differential Revision: https://phab.mercurial-scm.org/D12262

automation: make system3 the default for run-tests-linux We'll soon drop support for Python 2.7. Let's use Python 3 by default. Differential Revision: https://phab.mercurial-scm.org/D12261

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.