Prefer unittest.mock but require mock for python < 3.3.

The unit tests require mock but don't specify it, which causes
problems for any usages outside tox (e.g. testing that changes to
twisted don't break klein).  Use PEP508 to specify environment markers
and only install mock when required.  Tests can then conditionally
import unittest.mock and fall back to mock.

The conditional imports do raise a mypy error, discussed here:
    python/mypy#1153

So an annotation is required to silence this specific warning.

requirements.txt

@@ -0,0 +1 @@
+mock ; python_version <= '3.3'

src/klein/test/test_app.py

@@ -2,7 +2,10 @@
 
 import sys
 
-from mock import Mock, patch
+try:
+    from unittest.mock import Mock, patch
+except Exception:
+    from mock import Mock, patch  # type:ignore
 
 from twisted.python.components import registerAdapter
 from twisted.trial import unittest

src/klein/test/test_resource.py

@@ -3,7 +3,10 @@
 import os
 from io import BytesIO
 
-from mock import Mock, call
+try:
+    from unittest.mock import Mock, call
+except Exception:
+    from mock import Mock, call  # type:ignore
 
 from six.moves.urllib.parse import parse_qs