Allow tests that perform remote data access to be disabled

[drdavella]

This makes use of the pytest-remotedata plugin in order to control tests that require access to remote data. By default all tests will run, including those that access remote data. To disable this, pass --remote-data=none to the pytest command.

This means that pytest-remotedata is now a test dependency.

This resolves #399. cc @olebole

[olebole]

There is still one remote access in docs/index.rst:

045     >>> from astropy.io import fits
046     >>> from astropy import units as u
047     >>> import numpy as np
048     >>> from matplotlib import pyplot as plt
049     >>> from astropy.visualization import quantity_support
050     >>> quantity_support()  # for getting units on the axes below  # doctest: +IGNORE_OUTPUT
051 
052     Now we load the dataset from it's canonical source:
053 
054     >>> f = fits.open('https://dr14.sdss.org/optical/spectrum/view/data/format=fits/spec=lite?plateid=1323&mjd=5279
UNEXPECTED EXCEPTION: URLError(gaierror(-3, 'Temporary failure in name resolution'),)
T

[SaOgaz]

Can confirm that this ran correctly for me locally.

[drdavella]

@olebole do you have a chance to test this again?

[olebole]

Yes, I can confirm it works for me. Thank you for the fix.

[olebole]

Works for me.