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hghave: fix the definition of `python3` to work on Windows

Authored by mharbison72.

Description

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