Fix type errors; unbound mypy

[jwodder]

This silences some typing errors related in some inscrutable way to the latest version of retrying. It also unrestricts the maximum mypy version so that we can use the newly-released v1.0.

[codecov]

Codecov Report

Base: 89.16% // Head: 89.16% // No change to project coverage 👍

Coverage data is based on head (9095ddc) compared to base (67f015c).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1207   +/-   ##
=======================================
  Coverage   89.16%   89.16%           
=======================================
  Files          76       76           
  Lines        9468     9468           
=======================================
  Hits         8442     8442           
  Misses       1026     1026           

Flag	Coverage Δ
unittests	89.16% <100.00%> (ø)

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Impacted Files	Coverage Δ
dandi/metadata.py	87.55% <100.00%> (ø)

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[yarikoptic]

Thank you!

[github-actions]

🚀 PR was released in 0.49.0 🚀