Update astropy-helpers to v2.0.8

[bsipocz]

This is an automated update of the astropy-helpers submodule to v2.0.8. This includes both the update of the astropy-helpers sub-module, and the ah_bootstrap.py file, if needed.

A full list of changes can be found in the changelog.

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 v3.1.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-affiliated/helpers_3.py

[coveralls]

Coverage remained the same at 70.492% when pulling cef7c9f on bsipocz:update-helpers-v2.0.8 into 9e05d84 on pyvirtobs:master.