Fix isomorphic for molecular graphs

[rohithsrinivaas]

Closes #3164

[codecov-commenter]

Codecov Report

Patch coverage: 76.47% and project coverage change: -0.56% ⚠️

Comparison is base (5865c95) 73.40% compared to head (5c648e5) 72.85%.

❗ Current head 5c648e5 differs from pull request most recent head 23f4e5c. Consider uploading reports for the commit 23f4e5c to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3221      +/-   ##
==========================================
- Coverage   73.40%   72.85%   -0.56%     
==========================================
  Files         238      238              
  Lines       70251    70252       +1     
  Branches    16332    16332              
==========================================
- Hits        51570    51183     -387     
- Misses      15543    15965     +422     
+ Partials     3138     3104      -34     

Files Changed	Coverage Δ
pymatgen/analysis/graphs.py	74.77% <76.47%> (+0.02%)	⬆️

... and 6 files with indirect coverage changes

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