Fix CI/RTD failures (spacetelescope#3317)

* Add intersphinx ref

* ignore mpl_scatter_density deprecation warning

* specify mpl_scatter_density only

---------

Co-authored-by: Ricky O'Steen <rosteen@stsci.edu>

jdaviz/utils.py

@@ -364,7 +364,7 @@ def flux_conversion(values, original_units, target_units, spec=None, eqv=None, s
     spec : `~specutils.Spectrum1D`, optional
         The Spectrum1D object that will have converted flux or surface brightness units.
 
-    eqv : list of `astropy.units.equivalencies`, optional
+    eqv : list of :ref:`astropy:unit_equivalencies`, optional
         A list of Astropy equivalencies necessary for complex unit conversions/translations.
 
     slice : `astropy.units.Quantity`, optional

pyproject.toml

@@ -138,6 +138,8 @@ filterwarnings = [
     "ignore::DeprecationWarning:glue",
     "ignore::DeprecationWarning:asteval",
     "ignore:::specutils.spectra.spectrum1d",
+    # Remove the following line once https://github.com/astrofrog/mpl-scatter-density/issues/46 is addressed
+    "ignore:pkg_resources is deprecated as an API:DeprecationWarning:mpl_scatter_density",
     # Ignore numpy 2.0 warning, see https://github.com/astropy/astropy/pull/15495
     # and https://github.com/scipy/scipy/pull/19275
     "ignore:.*numpy\\.core.*:DeprecationWarning",