bpo-45506: Normalize _PyPathConfig.stdlib_dir when calculated. (#29040)

The recently added PyConfig.stdlib_dir was being set with ".." entries. When __file__ was added for from modules this caused a problem on out-of-tree builds. This PR fixes that by normalizing "stdlib_dir" when it is calculated in getpath.c.

https://bugs.python.org/issue45506

Lib/test/test_fileutils.py

@@ -0,0 +1,30 @@
+# Run tests for functions in Python/fileutils.c.
+
+import os
+import os.path
+import unittest
+from test.support import import_helper
+
+# Skip this test if the _testcapi module isn't available.
+_testcapi = import_helper.import_module('_testinternalcapi')
+
+
+class PathTests(unittest.TestCase):
+
+    def test_capi_normalize_path(self):
+        if os.name == 'nt':
+            raise unittest.SkipTest('Windows has its own helper for this')
+        else:
+            from .test_posixpath import PosixPathTest as posixdata
+            tests = posixdata.NORMPATH_CASES
+        for filename, expected in tests:
+            if not os.path.isabs(filename):
+                continue
+            with self.subTest(filename):
+                result = _testcapi.normalize_path(filename)
+                self.assertEqual(result, expected,
+                    msg=f'input: {filename!r} expected output: {expected!r}')
+
+
+if __name__ == "__main__":
+    unittest.main()