Fix type annotation of download_kwargs

[cthoyt]

Closes #145

This PR extends the RequestKwargs type dictionary with the other parts that are needed to call the pystow.utils.download() function

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.81%. Comparing base (fe6d8ba) to head (1c66762).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
+ Coverage   75.72%   75.81%   +0.09%     
==========================================
  Files          17       17              
  Lines        1767     1774       +7     
  Branches      211      211              
==========================================
+ Hits         1338     1345       +7     
  Misses        336      336              
  Partials       93       93              

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