Update test matrix

[mcara]

Updates the test matrix and also fixes an incompatibility with upcoming changes in astropy 7.0 - see astropy/astropy#16673 (comment)

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 93.63%. Comparing base (6044438) to head (7c63fec).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
tweakwcs/linearfit.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #207      +/-   ##
==========================================
- Coverage   93.73%   93.63%   -0.10%     
==========================================
  Files          11       11              
  Lines        2010     2011       +1     
==========================================
- Hits         1884     1883       -1     
- Misses        126      128       +2     

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[zacharyburnett]

this looks good, but do we need all of this specificity in numpy versions? Would it serve the same purpose to have one test for numpy 1.*, one for 2.*, and one for a nightly build?