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exewrapper: avoid directly linking against python3X.dll

Authored by mharbison72.

Description

exewrapper: avoid directly linking against python3X.dll

Subsequent code calls LoadLibrary() to attempt to load the DLL, but because of
this symbol reference, there is an attempt to load the DLL used during the build
prior to _main() running. This causes the whole process to fail if the DLL
isn't in the standard search path. That also means it will never load the DLL
for HackableMercurial. (Maybe we should get rid of that for py3, since you can
install python for a user without admin rights?)

This could also be resolved by calling GetProcAddress() on the symbol and
dereferencing it, but using the environment variable is consistent with the
*.bat file since fc8a5c9ecee0. (The environment variable persists after the
interpreter is initialized.)

Far more concerning is somehow I've gotten my system into a state where setting
the flag causes any output to the pager to be lost (as if it wasn't set at all)
in MSYS, cmd.exe, WSL, and PowerShell using py3.9.0, but the environment
variable works properly. I'm sure this flag worked on some versions of py3, so
I'm not sure what's going on here. This is might be related to init config
related changes in 3.8[1], since it works with 3.7.8, but fails with 3.8.1.
Somebody who understands encoding issues better than I do should give some
thought to if we need to make some changes to our encoding strategy on Windows
with py3.

With or without the flag/envvar, there is proper output if the command is
directly paged by piping to more.com (in any environment) or less (in MSYS
and WSL), or if paging is disabled with --pager=no. Legacy mode is required
though when Mercurial decides to spin up a pager.

[1] https://bugs.python.org/issue41941

Differential Revision: https://phab.mercurial-scm.org/D10756