Fix error for typing special forms

[jayqi]

Fixes runtime error when adding models that have fields annotated with certain typing special forms like Any and TypeVar instances.

Closes #114.

[github-actions]

🚀 Deployed on https://deploy-preview-115--nervous-visvesvaraya-4b7042.netlify.app

[codecov]

Codecov Report

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

Project coverage is 96.4%. Comparing base (e712754) to head (82b373b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #115     +/-   ##
=======================================
- Coverage   98.5%   96.4%   -2.1%     
=======================================
  Files         19      19             
  Lines        816     821      +5     
=======================================
- Hits         804     792     -12     
- Misses        12      29     +17     

Files	Coverage Δ
erdantic/core.py	97.3% <83.3%> (-0.8%)	⬇️

... and 8 files with indirect coverage changes