hghave: fix the definition of `python3` to work on Windows
Both py2 and py3 executables are named `python.exe`, and may or may not be on
PATH. So use the dispatcher executable that comes with py3 to fetch the version
of the latest py3 executable. This allows at least one relnotes test to run on
Windows.
Differential Revision: https://phab.mercurial-scm.org/D10694
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