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
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