Don't hardcode Python versions in test

As noted in change If8184c190e76d8cefb5b097f8fa8cb7564207103, the format
of an internal error message changed in Python 3. There's no need to
manually list each of these versions though - just ensure it's Python 3.

Change-Id: Ie1a97d251c6098054bdc963acf53c3182b52b2aa
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>
Closes-Bug: #1855493

nova/tests/unit/test_api_validation.py

@@ -14,7 +14,6 @@
 
 import copy
 import re
-import sys
 
 import fixtures
 from jsonschema import exceptions as jsonschema_exc
@@ -444,7 +443,7 @@ class PatternPropertiesTestCase(APIValidationTestCase):
         # Note(jrosenboom): This is referencing an internal python error
         # string, which is no stable interface. We need a patch in the
         # jsonschema library in order to fix this properly.
-        if sys.version[:3] in ['3.5', '3.6', '3.7']:
+        if six.PY3:
             detail = "expected string or bytes-like object"
         else:
             detail = "expected string or buffer"