switch fully to numpy.trapz and fix numpy version

[karllark]

Fixes three different test errors.

Numpy version fixed to 1.26.4 to avoid issues with upstream packages being compiled to different libraries (mismatch of variable sizes).

Remove last use of scipy.trapz by switching to numpy.trapz. Scipy seems to have renamed their function to trapezoid.

Updated codecov to v4 (from v1) to fix upload rate limit issue.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 41.92%. Comparing base (a66947b) to head (002945b).
Report is 29 commits behind head on master.

Files with missing lines	Patch %	Lines
beast/observationmodel/phot.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #815      +/-   ##
==========================================
- Coverage   42.00%   41.92%   -0.09%     
==========================================
  Files         108      108              
  Lines       10279    10264      -15     
==========================================
- Hits         4318     4303      -15     
  Misses       5961     5961              

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