setup: copy python3.dll next to hg.exe when building on Windows for hgext.git
I thought I took care of this already, but it must have been that I just
manually copied the file over locally when debugging why the pygit2 library
wasn't loading. The problem with that is what was copied over was from py38,
and then running a py39 build hard crashed when the extension was loaded.
Differential Revision: https://phab.mercurial-scm.org/D10301