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