pgsql: Provide for testing on python3 modules when under MSVC

Provide for testing on python3 modules when under MSVC

This should have been done some years ago as promised in commit
c4dcdd0c2. However, better late than never.

Along the way do a little housekeeping, including using a simpler test
for the python version being tested, and removing a redundant subroutine
parameter. These changes only apply back to release 9.5.

Backpatch to all live releases.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/966268c7621c4bca534961440b497a3270395fc2

Modified Files
--------------
src/tools/msvc/Install.pm   |  8 +---
src/tools/msvc/vcregress.pl | 94 +++++++++++++++++++++++++++++++--------------
2 files changed, 68 insertions(+), 34 deletions(-)