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.

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

We've also noticed that you likely used the astropy package template in the past for your package and that your conftest.py file is now out of date and imports pytest plugins from astropy:

from astropy.tests.pytest_plugins import *

This astropy sub-package has been removed, so your tests will likely be failing now that astropy v3.2 has been released. This PR therefore fixes this import by checking the astropy version and adjusting the import as needed.

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

[pep8speaks]

Hello @bsipocz! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file ah_bootstrap.py:

Line 124:80: E501 line too long (92 > 79 characters)
Line 126:80: E501 line too long (98 > 79 characters)
Line 133:80: E501 line too long (86 > 79 characters)
Line 136:80: E501 line too long (93 > 79 characters)
Line 156:80: E501 line too long (88 > 79 characters)

• In the file stingray/conftest.py:

Line 15:80: E501 line too long (84 > 79 characters)
Line 16:1: E402 module level import not at top of file

[matteobachetti]

Thanks @bsipocz. I made a slight change to .travis.yml to fix a build failure in the docs, now it looks good.