Apply assertions in debug mode only

[asvetlov]

A work on #2859

[codecov-io]

Codecov Report

Merging #2966 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2966      +/-   ##
==========================================
+ Coverage   97.98%   97.98%   +<.01%     
==========================================
  Files          40       40              
  Lines        7592     7594       +2     
  Branches     1323     1325       +2     
==========================================
+ Hits         7439     7441       +2     
  Misses         51       51              
  Partials      102      102

Impacted Files	Coverage Δ
aiohttp/web_urldispatcher.py	99.1% <100%> (ø)	⬆️
aiohttp/web_app.py	99.1% <100%> (ø)	⬆️

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[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new