fix test with warnings

[beliaev-maksim]

pipe on master failed: https://github.com/getsentry/responses/runs/5068593004?check_suite_focus=true

new version of pytest was released, now we cannot check warns with None, there is a new method:
https://docs.pytest.org/en/7.0.x/how-to/capture-warnings.html#additional-use-cases-of-warnings-in-tests

[codecov]

Codecov Report

Merging #493 (e5708f2) into master (2a9924e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #493   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines         2160      2161    +1     
=========================================
+ Hits          2160      2161    +1     

Impacted Files	Coverage Δ
responses/test_responses.py	100.00% <100.00%> (ø)

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