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.