Get a dropin started

We can put a plugin here later.

setup.cfg

@@ -23,9 +23,8 @@ platform = POSIX
 # dict with a key "" and a value "src".
 package_dir =
     =src
-# Use the automatic finder to pick up packages.  It is configured below.
+# Explicitly list our packages because the automatic finder can't deal with
+# the plugins package we want to ship.
 packages =
-    find:
-
-[options.packages.find]
-where = src
+    _secureaccesstokenauthorizer
+    twisted.plugins

src/twisted/plugins/secureaccesstokenauthorizer.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.
+
+"""
+A drop-in to supply plugins to the Twisted plugin system.
+"""