Make public types explicit for PyLance.

[dajiaji]

No description provided.

[codecov]

Codecov Report

Merging #180 (eb90617) into main (59a59a7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #180   +/-   ##
=======================================
  Coverage   99.84%   99.84%           
=======================================
  Files          28       28           
  Lines        2604     2610    +6     
  Branches      676      676           
=======================================
+ Hits         2600     2606    +6     
  Misses          4        4           

Impacted Files	Coverage Δ
cwt/cwt.py	100.00% <100.00%> (ø)

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