Add mypy and tidy up typings

[mikelane]

No description provided.

[codecov]

Codecov Report

Merging #5 (58737bb) into main (549318c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main        #5   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           93        94    +1     
=========================================
+ Hits            93        94    +1     

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

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