Make web.BaseRequest and web.Request slot-based

[asvetlov]

Adding __slots__ prevents wild objects modification, e.g request.custom = 123.
Both request and response objects support dict-like interface already (request['custom'] = 123).

[codecov-io]

Codecov Report

Merging #3942 into master will decrease coverage by 2.93%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3942      +/-   ##
==========================================
- Coverage   97.67%   94.73%   -2.94%     
==========================================
  Files          43       43              
  Lines        8761     8759       -2     
  Branches     1377     1375       -2     
==========================================
- Hits         8557     8298     -259     
- Misses         89      349     +260     
+ Partials      115      112       -3

Impacted Files	Coverage Δ
aiohttp/client_reqrep.py	96.99% <100%> (ø)	⬆️
aiohttp/helpers.py	96.55% <100%> (-0.98%)	⬇️
aiohttp/web_response.py	97.67% <100%> (ø)	⬆️
aiohttp/web_request.py	97.42% <100%> (+1.23%)	⬆️
aiohttp/worker.py	6.72% <0%> (-90.76%)	⬇️
aiohttp/resolver.py	45.61% <0%> (-54.39%)	⬇️
aiohttp/web_fileresponse.py	66.66% <0%> (-29.89%)	⬇️
aiohttp/tcp_helpers.py	66.66% <0%> (-26.67%)	⬇️
aiohttp/locks.py	92.85% <0%> (-7.15%)	⬇️
aiohttp/web_runner.py	92.47% <0%> (-5.31%)	⬇️
... and 8 more

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