automation: implement "publish-windows-artifacts" command The new command and associated functionality can be used to automate the publishing of Windows release artifacts. It supports uploading wheels to PyPI (using twine) and copying the artifacts to mercurial-scm.org and updating the latest.dat file to advertise them via the website. I ran `automation.py publish-windows-artifacts 5.1.1` and it appeared to "just work." But the real test will be to do this on the next release... Differential Revision: https://phab.mercurial-scm.org/D6786