pip wheel fails using --global-option build_ext

After the update to requirements.txt in 4.35.0 introduced by the fix to bug 1756352, oslo.db now ultimately requires unittest2 due to it including testresources. However, unittest2 won't build using current python build tools (this has been reported in multiple locations over a number of years and has not been fixed). The oldest report I found is here: https://github.com/pypa/setuptools/issues/409.

This means it is no longer possible to build oslo.db's required wheels from source using the supplied requirements, i.e. it is no longer possible to follow the methods here: https://pip.pypa.io/en/stable/user_guide/#installing-from-wheels to successfully build oslo.db and its requirements into a single tarball for later distribution.

Specifically, the following now fails:

1) create a virtual environment for python 3.5 (or 3.6) and activate
2) install current pip, setuptools, and wheel packages
3) Checkout oslo.db at tag 4.35.0
4) Run the pip wheel --global-option build_ext --wheel-dir=dist -r requirements.txt

Result:

The wheel build will fail at unittest2.

Expected results:

The wheel build will succeed and all required wheels for oslo.db will be in dist.