support astropy 5.x masked columns

[sbailey]

This PR updates desi_qso_mgii_afterburner to support astropy 5.x which (IMHO) incorrectly interprets FITS table blank strings as masked values instead of just passing forward blank strings. This is the same issue as the fibermap reading problem in PR #1454, but here I'm just fixing the impacted column (SUBTYPE) instead of bringing in an fitsio dependency. The relevant change is:

    if hasattr(redshifts['SUBTYPE'], 'mask'):
        redshifts['SUBTYPE'][redshifts['SUBTYPE'].mask] = ''

Without this change, desi_qso_mgii_afterburner fails with:

Traceback (most recent call last):
  File "/global/common/software/desi/users/malvarez/desiconda2/code/desispec/master/bin/desi_qso_mgii_afterburner", line 265, in <module>
    QSO_from_MGII = select_targets_with_mgii_fitter(redrock, fibermap, sel_to_mgii, args.coadd, args.redrock, param_mgii_fitter, DESI_TARGET, args.save_target)
  File "/global/common/software/desi/users/malvarez/desiconda2/code/desispec/master/bin/desi_qso_mgii_afterburner", line 86, in select_targets_with_mgii_fitter
    index_with_mgii_fit) = mgii_fitter(spectra_name, redrock_name, sel_to_mgii,
  File "/global/common/software/desi/users/malvarez/desiconda2/code/desispec/master/py/desispec/mgii_afterburner.py", line 401, in mgii_fitter
    index_with_fit) = get_spectra(spectra_name,
  File "/global/common/software/desi/users/malvarez/desiconda2/code/desispec/master/py/desispec/mgii_afterburner.py", line 174, in get_spectra
    model_wave, model_flux = create_model(spectra,
  File "/global/common/software/desi/users/malvarez/desiconda2/code/desispec/master/py/desispec/mgii_afterburner.py", line 93, in create_model
    tx    = templates[(zb['SPECTYPE'], zb['SUBTYPE'])]
TypeError: unhashable type: 'MaskedConstant'

While I'm at it, I also updated some 2-space indentation to 4-space indentation to match our code style.

Thanks for @marcelo-alvarez for the pipeline testing with updated dependencies that identified this.