Fix mypy/pyright errors in serde/se.py

[yukinarit]

Closes #397

[codecov]

Codecov Report

Patch coverage: 85.00% and project coverage change: -0.08% ⚠️

Comparison is base (07c8374) 89.79% compared to head (8c23a5c) 89.71%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #420      +/-   ##
==========================================
- Coverage   89.79%   89.71%   -0.08%     
==========================================
  Files          12       12              
  Lines        1891     1896       +5     
  Branches      388      390       +2     
==========================================
+ Hits         1698     1701       +3     
- Misses        142      143       +1     
- Partials       51       52       +1     

Files Changed	Coverage Δ
serde/se.py	97.48% <76.92%> (-0.62%)	⬇️
serde/compat.py	83.79% <100.00%> (+0.11%)	⬆️
serde/core.py	85.17% <100.00%> (ø)

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