pip has recently started to compare wheel tags with system supported wheel
tags, and as it turns out build tools just use sysconfig.get_platform() to generate
a tag (in setuptools, wheel, packaging, etc) for non-official platforms. For
official platforms they do various normalization steps and use other sources.
Since we can't really patch all those packages we need to provide a platform
string that wont change between updates.
* Don't include the cygwin version in it anymore, since that can change on updates.
* Hardcode to "cygwin" since builing for cygwin can change the output of os.uname()
The later breaks detection of MSYS2 compared to cygwin, but ideally no one should
depend on it, and we want to get rid of the differences there anyway.
Fixes#5143
425bc5adb3