setup.py: Get prefix directly from sys instead of distutils

[hugovk]

For #4796.

distutils

setup.py does:

from distutils import sysconfig
prefix = sysconfig.get_config_var("prefix")

https://github.com/python/cpython/blob/master/Lib/distutils/sysconfig.py

get_config_var("prefix") calls get_config_vars().get("prefix"), which initialised it as
_config_vars['prefix'] = PREFIX, where PREFIX = os.path.normpath(sys.prefix)

sysconfig

There's also a sysconfig module in the stdlib:

• https://docs.python.org/3/library/sysconfig.html

This is almost identical to the one in distutils:

get_config_var("prefix") calls get_config_vars().get("prefix"), which initialised it as
_CONFIG_VARS['prefix'] = _PREFIX = _PREFIX, where PREFIX = os.path.normpath(sys.prefix)

https://github.com/python/cpython/blob/master/Lib/sysconfig.py

I tried our setup.py with:

-from distutils import sysconfig
+import sysconfig
 prefix = sysconfig.get_config_var("prefix")

Which passed on all CI jobs except MSYS MINGW32 and MINGW64 because the drive letter changed:

distutils sysconfig prefix:   C:/msys64/mingw64
stdlib    sysconfig prefix:   D:/msys64/mingw64

os.path.normpath(sys.prefix)

I didn't dig into why they're different, but can we skip distutils.sysconfig and sysconfig altogether and use os.path.normpath(sys.prefix) directly?

sys.prefix:                   C:/msys64/mingw64
os.path.normpath(sys.prefix): C:/msys64/mingw64

[radarhere]

I've created hugovk#57 as a suggestion