packaging: remove requirements constraints to support Python 2 We just deleted support for Python 2 from the packaging code. We no longer need these package constraints in the requirements file to support Python 2. Differential Revision: https://phab.mercurial-scm.org/D12267

packaging: remove py2exe / Python 2.7 support This commit started by deleting references to py2exe (which is only used on Python 2). After pulling the thread, quite a lot of code was orphaned and was deleted. Differential Revision: https://phab.mercurial-scm.org/D12265

automation: delete code related to Python 2.7 support The building of Inno and WiX installers took a python_version argument that allowed us to specify "2" or "3" for the major Python version. Since we no longer support Python 2, we can delete this argument and everything feeding into it. Differential Revision: https://phab.mercurial-scm.org/D12264

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