automation: support building Windows wheels for Python 3.7 and 3.8 The time has come to support Python 3 on Windows. Let's teach our automation code to produce Windows wheels for Python 3.7 and 3.8. We could theoretically support 3.5 and 3.6. But I don't think it is worth it. People on Windows generally use the Mercurial installers, not wheels. And I'd prefer we limit variability and not have to worry about supporting earlier Python versions if it can be helped. As part of this, we change the invocation of pip to `python.exe -m pip`, as this is what is being recommended in Python docs these days. And it seemed to be required to avoid a weird build error. Why, I'm not sure. But it looks like pip was having trouble finding a Visual Studio files when invoked as `pip.exe` but not when using `python.exe -m pip`. Who knows. Differential Revision: https://phab.mercurial-scm.org/D8478