[issue39406] Implement os.putenv() with setenv() if available

[STINNER Victor]

STINNER Victor <vstinner at python.org> added the comment:

> Maybe use SetEnvironmentVariable() on Windows?

I didn't know this function. I updated PR 18095.

----------

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39406>
_______________________________________