Fix typing problems found by MyPy

[pont-us]

Fixes #68

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.40%. Comparing base (55387fc) to head (afd9360).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
xcengine/core.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
- Coverage   84.46%   84.40%   -0.06%     
==========================================
  Files           6        6              
  Lines         547      558      +11     
  Branches       64       64              
==========================================
+ Hits          462      471       +9     
- Misses         78       80       +2     
  Partials        7        7              

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