Astropy: Remove deprecated functionality for 4.0 release

[pllim]

As discussed on Slack, this issue is to track the removal of deprecated stuff for astropy 4.0 LTS release. Update this as needed. I added cc for people who brought it up or implemented the deprecation or would be the most interested in its future, but all are welcome to have a look.

Disclaimer: I found the following using a simple grep for "deprecate". If I missed something, feel free to add to the list.

Candidates for removal

(Listing more or less in the alphabetical order of the affected sub-package. Check the box if removal is done and xref the relevant PRs.)

• Remove io.ascii Reader/Writer and unified I/O interfaces; see #8567 -- cc @taldcroft (UPDATE: Pushed back due to lack of time)
• [x] Remove deprecated keyword in convolve.py, which was done in 2.0 release in #5782 -- cc @keflavich See #9356
• [x] Remove deprecated parameter from Gaussian2DKernel, which was done in 3.0 release in #6748 -- cc @adamginsburg See #9356
• Remove representation keyword in coordinates baseframe.py, which was deprecated in 3.1 release in #8119 -- cc @eteq _EDIT: this was punted to a future LTS release_
• [x] Remove deprecated arguments in galactocentric.py, which was done in 2.0 release in #6219 -- cc @adrn Removed in #9326
• [x] Remove latitude and longitude in earth.py, which was deprecated in 2.0 release in #6237 -- cc @eteq Removed in #9326
• [X] Remove recommended_units in representations.py, which was deprecated in 3.0 release in #6858 -- cc @mhvk (#8892)
• [x] Remove compatibility_mode in hdf5.py, which was deprecated in 3.0 release in #6304 -- cc @matteobachetti Done in #8899
• [x] Remove usecPickle in pickle_helpers.py, which was deprecated in 3.0 release in #6655 -- cc @bsipocz. Removed in #8890
• [x] Remove rotation_matrix_from_angle method in rotations.py, which was deprecated in 2.0 release in #6235 -- @nden Removed in #9369
• [x] Remove deprecated_renamed_argument from biweight.py, which was added in 2.0 release in #5991 -- @larrybradley. Removed in #8948.
• [x] Remove deprecated_renamed_argument from stats/funcs.py, which was added in 2.0 release in #5232 -- cc @keflavich. Removed in #9011.
• [x] Remove deprecated_renamed_argument from sigma_clipping.py, which was added in 3.1 release in #7478 -- cc @larrybradley. Removed in #8948.
• [X] Remove magnitude_zero_points.py, which was deprecated in 3.1 release in #7891 -- see #9353
• [x] Remove funcsigs.py and futures, which were deprecated in 3.0 release in #6598 -- cc @saimn. Done in #8909
• [x] Remove utils.compat.numpy modules that were deprecated in 3.0 release in #6691 -- cc @MSeifert04 Done in #8910
• [x] Remove _naxis1 and _naxis2 from wcs.py, which were deprecated in 3.1 in #7973 -- cc @nden Removed in #9465.
• [x] Remove bundled six, which is not needed after 3.0 release -- PR by @saimn waiting at #8315, see https://github.com/astropy/astropy/issues/8888#issuecomment-504146773

Need clarification

• Deprecate representation, remove _normalize_representation_type; see #8886 -- cc @adrn _EDIT: this was clarified in #9320_

Decide to keep for now

• Anything related to io.fits 😬
• Anything in extern; Looking at you, jsquery and configobj.
• Deprecated stats.bls and stats.lombscargle namespace, which was moved in 3.2 in #8599 -- https://github.com/astropy/astropy/issues/8888#issuecomment-504137880

[bsipocz]

Remove deprecated stats.bls and stats.lombscargle namespace, which was moved in 3.2 in #8599

this is too soon, anecdotally we know that many people are using LS from astropy both in astronomy and beyond.

[bsipocz]

@pllim - we also need to remove six, that's in extern and the PR is already been waiting for the merge button to be pushed (but first we rather make sure again that none of the coordinated packages, etc are using it in their latest releases...).

[mhvk]

A very lucky number to remove cruft!

Aside: previously, it seems we never added entries to the changelog for removals of deprecated features. Should we?

[bsipocz]

We did, usually under API change, or other changes, starting with the word "Removed".

[bsipocz]

@nden - Do you think the deprecations listed above could be removed from modeling and wcs? If yes, I'm happy to go ahead and open PRs for them, otherwise we can remove them from the list.

[pllim]

@bsipocz , I think we done here! If you agree, please close the issue. 😄

[bsipocz]

I found this issue super helpful to keep track of stuff we needed to clean up, and vote for having similar ones for new releases, or at least for the new LTS releases, too.

[mhvk]

Indeed, this was super helpful, thanks @pllim!