tests: drop dependency on external mock module for newer python (#574)

Fixes #541

Co-authored-by: Łukasz Langa <lukasz@langa.pl>

html5lib/tests/test_meta.py

@@ -1,7 +1,10 @@
 from __future__ import absolute_import, division, unicode_literals
 
 import six
-from mock import Mock
+try:
+    from unittest.mock import Mock
+except ImportError:
+    from mock import Mock
 
 from . import support
 

requirements-test.txt

@@ -6,6 +6,5 @@ pytest>=4.6.10,<5 ; python_version < '3'
 pytest>=5.4.2,<8 ; python_version >= '3'
 coverage>=5.1,<6
 pytest-expect>=1.1.0,<2
-mock>=3.0.5,<4 ; python_version < '3.6'
-mock>=4.0.2,<5 ; python_version >= '3.6'
+mock>=3.0.5,<4 ; python_version < '3.3'
 setuptools; python_version >= '3.12'