Small typing fixes

[ianyfan]

Description

This fixes some typing issues that arose on a fresh install:

• pytype fails in preference_comparison.py:

  imitation/src/imitation/algorithms/preference_comparisons.py

  Line 773 in 944edce

  return var_estimate

  since var_estimate is defined in the previous code branches, except one, which calls raise_uncertainty_on_not_supported which raises an error, and this requires an explicit type annotation to indicate that
• mypy should be updated to version 0.981 since we don't target Python 3.7 (ref Allow stubs to use newer syntax than 3.7 python/mypy#13500)

This is a very small PR, so I can add more typing fixes where I notice them to add some more substance, if desired.

Testing

ci/code_checks.sh, which failed on a fresh install, now passes

[codecov]

Codecov Report

Merging #590 (3883d10) into master (944edce) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #590   +/-   ##
=======================================
  Coverage   97.48%   97.48%           
=======================================
  Files          85       85           
  Lines        8105     8105           
=======================================
  Hits         7901     7901           
  Misses        204      204           

Impacted Files	Coverage Δ
src/imitation/algorithms/preference_comparisons.py	99.29% <100.00%> (ø)

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[AdamGleave]

LGTM.

Small PRs are fine -- preferable, in fact, easy to review. 3 lines is admittedly on the smaller end of a small PR, but feel free to just send these across as they come up.

Not sure why pytype was failing for you bot not on CI; I'm guessing version issue? But adding NoReturn seems like the right thing to do regardless.