Remove gzip compatibility layer since we now no longer support Python 2.6 - #4464

@embray - can you review this since it makes some changes in astropy.io.fits?

Looks good to me. I would ask the same question again about making our compat gzip module raise an exception and make sure tests pass.

One question also about astropy/tests/tests/test_imports.py. In your branch this still has references to the removed modules. Is that intended?

# Compatibility subpackages that should only be used on Python 2
_py2_packages = set([
    'astropy.extern.configobj_py2',
    'astropy.utils.compat._fractions_py2',
    'astropy.utils.compat._gzip_py2',
    'astropy.utils.compat._odict_py2',
    'astropy.utils.compat._subprocess_py2'
])

# Same but for Python 3
_py3_packages = set([
    'astropy.extern.configobj_py3',
    'astropy.utils.compat._gzip_py3'
])

@taldcroft - I did check that no test failures occurred if I raise an exception. I'll clean up the list of modules in test_imports - good catch (but not critical)

Much better....

@embray - done. Ready to merge? :)