Update import path for GriddedPSFModel

[bmorris3]

Importing GriddedPSFModel directly from photutils without specifying the psf module has been deprecated since astropy/photutils#1433, which causes WebbPSF calls to warn:

webbpsf/gridded_library.py:108: DeprecationWarning: `photutils.GriddedPSFModel` is a 
deprecated alias for `photutils.psf.GriddedPSFModel` and will be removed in the future.
Instead, please use `from photutils.psf import GriddedPSFModel` to silence this warning.

This PR first tries to import GriddedPSFModel from the psf module, and falls back on old the top-level import if that fails. Failing both of these, the ImportError is raised.

[obi-wan76]

This looks good, thanks!

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (b219a7d) 55.97% compared to head (af28236) 55.97%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #696   +/-   ##
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  Coverage    55.97%   55.97%           
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  Files           15       15           
  Lines         6280     6280           
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  Hits          3515     3515           
  Misses        2765     2765           

Files Changed	Coverage Δ
webbpsf/gridded_library.py	77.10% <100.00%> (ø)

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