hg: add user-site to `sys.path` on Windows to allow pip-installed extensions This has been in the TortoiseHg builds for several cycles now on Windows, and even longer on macOS. It allows an extension to be configured with `ext =` syntax, instead of requiring the full path to be specified. It's confusing for a user to be hit with messages about not being able to load extensions, based solely on which `hg.exe` is being run. This only applies to py2exe binaries, since wrapper.exe already sees into the user site area. There are no frozen binaries on other platforms (that I'm aware of), and an equivalent change will need to be made to `dispatch.py` in order to work with PyOxidizer, since it bypasses this module completely. (It also has the ability to use the `site` module, so it will look completely different.) Differential Revision: https://phab.mercurial-scm.org/D9531

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