[issue39413] Implement os.unsetenv() on Windows

[STINNER Victor]

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

PR 18107 still fix this issue (add os.unsetenv() to Python 3.9 on Windows), but fix the implementation (use _wputenv() rather than SetEnvironmentVariableW(), to update the CRT).

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39413>
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