bpo-39413: Implement os.unsetenv() on Windows (GH-18163)

The os.unsetenv() function is now also available on Windows.
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 4fec647..de3e560 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -645,6 +645,9 @@
 
    .. availability:: most flavors of Unix, Windows.
 
+   .. versionchanged:: 3.9
+      The function is now also available on Windows.
+
 
 .. _os-newstreams: