Prepare to reorganize upb file structure

I am getting ready to move almost everything under the upb/ directory up one
level to integrate upb better into its new location in the protobuf repo. This
change makes a few tweaks to prepare for that:
- Delete upb's LICENSE and CONTRIBUTING.md files since we already have similar
  files at the top level.
- Rename `//python:python_version` so that it won't conflict later with
  `//upb/python:python_version`.
- Move the contents of python/BUILD.bazel out to a Bazel macro to facilitate
  merging that BUILD.bazel file with upb/python/BUILD.

PiperOrigin-RevId: 567119840

python/python_version_test.py

@@ -0,0 +1,29 @@
+# Protocol Buffers - Google's data interchange format
+# Copyright 2008 Google Inc.  All rights reserved.
+#
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file or at
+# https://developers.google.com/open-source/licenses/bsd
+"""Test that Kokoro is using the expected version of python."""
+
+import os
+import sys
+import unittest
+
+
+class PythonVersionTest(unittest.TestCase):
+
+  def testPython3(self):
+    """Test that we can import nested import public messages."""
+
+    exp = os.getenv('KOKORO_PYTHON_VERSION', '')
+    if not exp:
+      print('No kokoro python version found, skipping check', file=sys.stderr)
+      return
+    self.assertTrue(
+        sys.version.startswith(exp),
+        'Expected Python %s but found Python %s' % (exp, sys.version))
+
+
+if __name__ == '__main__':
+  unittest.main()