Avoid symbol clashes in plot_expressions

[dweindl]

Ignore sympy constants when sympifying expression strings to avoid clashes with sympy.*.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.20%. Comparing base (4eb4bbd) to head (508dc1f).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2440      +/-   ##
===========================================
- Coverage    77.64%   76.20%   -1.44%     
===========================================
  Files          324      319       -5     
  Lines        20928    20767     -161     
  Branches      1464     1464              
===========================================
- Hits         16249    15826     -423     
- Misses        4676     4937     +261     
- Partials         3        4       +1     

Flag	Coverage Δ
cpp	73.40% <100.00%> (-0.02%)	⬇️
cpp_python	34.09% <0.00%> (ø)
petab	?
python	72.17% <100.00%> (-0.01%)	⬇️

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Files	Coverage Δ
python/sdist/amici/numpy.py	87.61% <100.00%> (ø)

... and 21 files with indirect coverage changes