Fix to find Python files for Windows virtualenvs
Fix to find Python files for Windows virtualenvs

Virtualenvs do not have their Python DLLs etc in the `sys.prefix`
directory, but in the `sys.base_prefix` directory.  This directory is
the same as `sys.prefix` if not in a virtualenv, so is safe for either
case:

https://docs.python.org/3/library/sys.html#sys.base_prefix

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