Replace attrs with dataclasses

[asvetlov]

aiohttp 4.0 supports Python 3.7+.
There is no need to use attrs but better to switch to the standard dataclasses module.
attrs is great but the standard is even better.

The only downside is: for attrs you should use dataclasses.replace(ds, name=val) instead of attr.evolve(ds, name=val).
I think it is a good compromise.

[codecov]

Codecov Report

Merging #5284 (7949798) into master (7f73270) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5284      +/-   ##
==========================================
- Coverage   97.52%   97.52%   -0.01%     
==========================================
  Files          43       43              
  Lines        8784     8781       -3     
  Branches     1410     1410              
==========================================
- Hits         8567     8564       -3     
  Misses        103      103              
  Partials      114      114              

Flag	Coverage Δ
unit	97.41% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
aiohttp/client_reqrep.py	97.38% <100.00%> (-0.02%)	⬇️
aiohttp/connector.py	96.48% <100.00%> (ø)
aiohttp/helpers.py	96.78% <100.00%> (ø)
aiohttp/tracing.py	100.00% <100.00%> (ø)
aiohttp/web_request.py	97.60% <100.00%> (ø)
aiohttp/web_routedef.py	98.13% <100.00%> (ø)

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[gleb-chipiga]

As far as I understand, dataclasses does not support __slots__, which were used with attrs. Have you decided to stop using __slots__?

[asvetlov]

I just don't care.
Slots save some memory.
The difference was huge for old Python, with a new compact dict (which split keys and values structure) implemented a gap is negligible.

[Tinche]

@asvetlov You are free to do as you like, obviously, but dataclasses aren't better than attrs. Their functionality is a strict subset; it's mostly for use within the standard library and if you don't have access to third party packages. Everyone using aiohttp obviously has access to third party packages.