JP-2808: Fix handling of SRCTYPE in master background

[hbushouse]

Resolves JP-2808

This PR fixes the retrieval of the SRCTYPE value in the master_background step for the case of NIRSpec IFU data. The input background spectra (from x1d products) are always in the form of a SpecModel or MultiSpecModel, in which case the SRCTYPE must be read from the "spec" attribute of the data model, not from the generic model.meta.target.source_type attribute (which is None for these kinds of models).

Checklist for maintainers

• [x] added entry in CHANGES.rst within the relevant release section
• [ ] updated or added relevant tests
• [ ] updated relevant documentation
• [x] added relevant milestone
• [x] added relevant label(s)
• [x] ran regression tests, post a link to the Jenkins job below. How to run regression tests on a PR
• [ ] Make sure the JIRA ticket is resolved properly

[codecov[bot]]

Codecov Report

Merging #6980 (f44e41d) into master (abd9b56) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #6980   +/-   ##
=======================================
  Coverage   79.27%   79.27%           
=======================================
  Files         414      414           
  Lines       37459    37459           
=======================================
  Hits        29695    29695           
  Misses       7764     7764           

Flag	Coverage Δ		*Carryforward flag
nightly	79.24% <100.00%> (ø)		Carriedforward from e03c028
unit	53.11% <0.00%> (ø)

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
jwst/master_background/master_background_step.py	75.17% <100.00%> (ø)

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[hbushouse]

Clean regtest run at https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/384/

[hbushouse]

Ready for review. @dmggh @jemorrison @stscieisenhamer