Fix mypy local fails in tests/test_deprecated.py and tests/test_experimental.py

[HideakiImamura]

Motivation

This PR aims to fix the mypy . command failure in the local environment.

• Before

• After

Description of the changes

• Add several assertions to ensure the message is Warning.

[HideakiImamura]

@keisuke-umezawa @hvy Could you review this PR if you have time?

[codecov-io]

Codecov Report

Merging #2191 (5e555ff) into master (b4595a6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2191   +/-   ##
=======================================
  Coverage   92.09%   92.09%           
=======================================
  Files         124      124           
  Lines       10312    10312           
=======================================
  Hits         9497     9497           
  Misses        815      815           

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[hvy]

LGTM!

So, this happens with the later versions of pytest (not with e.g. v5), and it doesn't manifest in the CI since pytest isn't installed for the type checking job. Is this understanding correct?

[keisuke-umezawa]

LGTM!