Update astropy-helpers to v3.2.1

[bsipocz]

This is an automated update of the astropy-helpers submodule to v3.2.1. This includes both the update of the astropy-helpers sub-module, and the ah_bootstrap.py file, if needed.
Please note that since v3.2, sphinx-astropy needs to be explicitely listed as a dependency for building the documentation.
A full list of changes can be found in the changelog.

We also wanted to let you know that we have made new releases of the astropy package-template. You can find the latest cookiecutter template here or if you prefer you can find a rendered version of the template here.

This is intended to be helpful, but if you would prefer to manage these updates yourself, or if you notice any issues with this automated update, please let @bsipocz or @astrofrog know!

Similarly to the core package, the v3.0+ releases of astropy-helpers require Python 3.5+. We will open automated update PRs with astropy-helpers v3.2.x only for packages that specifically opt in for it when they start supporting Python 3.5+ only. Please let @bsipocz or @astrofrog know or add your package to the list in https://github.com/astropy/astropy-procedures/blob/master/update-packages/helpers_3.py

[mwcraig]

Thanks 🤖!