Add stub setup.py for editable installations.

In order to run:

    pip install -e .[docs,tests]

We found that you either need to add a stub setup.py like this or be
using a very recent version of pip. In order to streamline the
installation process on more machines with a relaxed pip version
constraint, this stub is added.

Fixes terrapower#1639.

setup.py

@@ -0,0 +1,17 @@
+# Copyright 2024 TerraPower, 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.
+"""Stub setup.py to allow editable installations on older pips."""
+from setuptools import setup
+
+setup()