Bring in Twisted for trial and add a stub test module.

requirements.txt

 # Pinned version of all dependencies, direct and transitive, needed at
 # runtime.  This is the configuration tested on CI.
+attrs==19.1.0
+Automat==0.7.0
+constantly==15.1.0
+hyperlink==19.0.0
+idna==2.8
+incremental==17.5.0
+PyHamcrest==1.9.0
+six==1.12.0
+Twisted==19.2.1
+zope.interface==4.6.0

src/_secureaccesstokenauthorizer/tests/test_tokens.py

+# Copyright 2019 PrivateStorage.io, LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""
+Some tests for some tokens, perhaps.  Just a stub for now.
+"""