Preserve asdf.fits_embed import

[andykee]

Fresh installs of HCIPy won't import due to a ModuleNotFoundError:

File [~/Sys/miniconda3/lib/python3.11/site-packages/hcipy/__init__.py:5]
      [1] # flake8: noqa
      [2]
      [3] # Import all submodules.
      [4] from . import aperture
----> [5] from . import atmosphere
      [6] from . import config
      [7] from . import coronagraphy

File [~/Sys/miniconda3/lib/python3.11/site-packages/hcipy/atmosphere/__init__.py:18]
      [1] __all__ = [
      [2]   'MultiLayerAtmosphere',
      [3]   'AtmosphericLayer',
   (...)
     [15]   'make_las_campanas_atmospheric_layers'
     [16] ]
---> [18] from .atmospheric_model import *
     [19] from .finite_atmospheric_layer import *
...
----> [6] import asdf.fits_embed
      [7] from astropy import wcs
      [8] from astropy.io import fits

ModuleNotFoundError: No module named 'asdf.fits_embed'

ASDF has deprecated the fits_embed module in v3.0: https://asdf.readthedocs.io/en/3.0.1/asdf/deprecations.html#asdf-in-fits-deprecation

This commit pins the ASDF version to < 3.0 to keep HCIPy installable. In the longer term, it would probably be advisable to migrate to stdatamodels (the recommended replacement for asdf.fits_embed)

[syhaffert]

This issue was also raised by #218 and was fixed by PR #199. The fix will be included in the next pip version release. @andykee the current work around is to work with the latest hcipy version on Github. Therefore, this PR is not necessary and I will close it.

[ehpor]

Agreed.