Make Key private.

[dajiaji]

No description provided.

[codecov]

Codecov Report

Merging #100 (7b8d8e8) into main (01c0f9a) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
- Coverage   99.58%   99.58%   -0.01%     
==========================================
  Files          24       24              
  Lines        1918     1917       -1     
  Branches      502      502              
==========================================
- Hits         1910     1909       -1     
  Misses          6        6              
  Partials        2        2              

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

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