[3.12] gh-111841: Fix os.putenv() and os.unsetenv() with embedded NUL on Windows (GH-111842) (GH-111966)

(cherry picked from commit 0b06d2482d77e02c5d40e221f6046c9c355458b2)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>