bpo-39413: Implement os.unsetenv() on Windows (GH-18104)

The os.unsetenv() function is now also available on Windows.

It is implemented with SetEnvironmentVariableW(name, NULL).

Modules/posixmodule.c

@@ -10163,7 +10163,49 @@ os_putenv_impl(PyObject *module, PyObject *name, PyObject *value)
 #endif /* HAVE_PUTENV */
 
 
-#ifdef HAVE_UNSETENV
+#ifdef MS_WINDOWS
+/*[clinic input]
+os.unsetenv
+    name: unicode
+    /
+
+Delete an environment variable.
+[clinic start generated code]*/
+
+static PyObject *
+os_unsetenv_impl(PyObject *module, PyObject *name)
+/*[clinic end generated code: output=54c4137ab1834f02 input=4d6a1747cc526d2f]*/
+{
+    /* PyUnicode_AsWideCharString() rejects embedded null characters */
+    wchar_t *name_str = PyUnicode_AsWideCharString(name, NULL);
+    if (name_str == NULL) {
+        return NULL;
+    }
+
+    BOOL ok = SetEnvironmentVariableW(name_str, NULL);
+    PyMem_Free(name_str);
+
+    if (!ok) {
+        return PyErr_SetFromWindowsErr(0);
+    }
+
+    /* Remove the key from posix_putenv_garbage;
+     * this will cause it to be collected.  This has to
+     * happen after the real unsetenv() call because the
+     * old value was still accessible until then.
+     */
+    if (PyDict_DelItem(_posixstate(module)->posix_putenv_garbage, name)) {
+        /* really not much we can do; just leak */
+        if (!PyErr_ExceptionMatches(PyExc_KeyError)) {
+            return NULL;
+        }
+        PyErr_Clear();
+    }
+
+    Py_RETURN_NONE;
+}
+/* repeat !defined(MS_WINDOWS) to workaround an Argument Clinic issue */
+#elif defined(HAVE_UNSETENV) && !defined(MS_WINDOWS)
 /*[clinic input]
 os.unsetenv
     name: FSConverter