DM-29835: Change the python type for FocalPlaneBackground

[timj]

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Merging #717 (eac74c0) into main (8116c1c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #717   +/-   ##
=======================================
  Coverage   84.59%   84.59%           
=======================================
  Files         243      243           
  Lines       31922    31922           
  Branches     5990     5990           
=======================================
  Hits        27006    27006           
  Misses       3746     3746           
  Partials     1170     1170           

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