Fix invalid type error when loading antimony model from file

[dweindl]

pathlib.Path -> str

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (51b0219) 77.75% compared to head (06c028f) 77.75%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2281   +/-   ##
========================================
  Coverage    77.75%   77.75%           
========================================
  Files          318      318           
  Lines        20510    20510           
  Branches      1436     1436           
========================================
  Hits         15947    15947           
  Misses        4560     4560           
  Partials         3        3           

Flag	Coverage Δ
cpp	73.53% <0.00%> (ø)
cpp_python	33.99% <0.00%> (ø)
petab	36.39% <0.00%> (ø)
python	72.30% <0.00%> (ø)
sbmlsuite	?

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Files	Coverage Δ
python/sdist/amici/antimony_import.py	94.11% <0.00%> (ø)