Here are a couple of patches, one for master and one for the back branches from 9.5 to 10 to allow testing of plpython3 and associated contrib modules when building with MSVC.
I've tested this out on a Windows 2016 machine with Visual Studio 2017.
I'd like to apply these - they only affect the testing script vcregress.pl, so they should be very low risk.
cheers
andrew
-- Andrew Dunstan https://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services