WIP: Add type hints everywhere

[asvetlov]

Enable strong mypy checks to make sure that types are correct.

[kornicameister]

@asvetlov I've added #3284 (not working yet, but wanted to fix up later) , but given this PR I assume mine overlaps yours. I think I will close my down unless you have other idea?

[asvetlov]

I've merged your PR into master and this branch.
Thank for your work.

This PR enables mypy checks in strongest possible mode.
I hope this will allow getting rid of typing errors, especially improper annotations.

[kornicameister]

@asvetlov happy to help. I am quite interested in getting this in since I am using both mypy and aiohttp in my project ;-). Those little errors in Neovim aren't necessary helping :D.

I will check out your PR and see if there's anything more to work on.
Or maybe I will find sth else ;-)

[kornicameister]

@asvetlov If you want, might be that #3289 is something you might want to pull in as well. I am not sure how the customs are here, but I usually prefer latest available libraries ;-).

[asvetlov]

@kornicameister if you want to help we can do the following:

1. I'll disable strict mypy mode (comment strict options in config) and merge the PR
2. You can pick up the master branch, uncomment these settings and work on some file
3. Your PR(s) will fail on CI, I'll check it manually and merge.

Adding types to all files is a big work, I appreciate any help

[kornicameister]

Yeah...why not 🤔. I will try to get sth as soon as I have some time off being dad 😃.

That said I will wait for you to merge this one. I suppose I can actually pick up this branch as a starting point for the being.

[kornicameister]

@asvetlov FYI: starting with aiohttp/payload.py

[asvetlov]

@kornicameister cool!
I'm about to make the PR green

[codecov-io]

Codecov Report

Merging #3280 into master will increase coverage by 0.03%.
The diff coverage is 99.01%.

@@            Coverage Diff             @@
##           master    #3280      +/-   ##
==========================================
+ Coverage   97.99%   98.03%   +0.03%     
==========================================
  Files          43       43              
  Lines        7896     8001     +105     
  Branches     1354     1354              
==========================================
+ Hits         7738     7844     +106     
  Misses         65       65              
+ Partials       93       92       -1

Impacted Files	Coverage Δ
aiohttp/web_protocol.py	92.02% <ø> (+0.27%)	⬆️
aiohttp/locks.py	100% <100%> (ø)	⬆️
aiohttp/resolver.py	100% <100%> (ø)	⬆️
aiohttp/http_writer.py	98.09% <100%> (+0.01%)	⬆️
aiohttp/pytest_plugin.py	98.63% <100%> (ø)	⬆️
aiohttp/cookiejar.py	100% <100%> (ø)	⬆️
aiohttp/abc.py	100% <100%> (ø)	⬆️
aiohttp/test_utils.py	99.27% <100%> (ø)	⬆️
aiohttp/streams.py	99.2% <100%> (ø)	⬆️
aiohttp/http_exceptions.py	100% <100%> (ø)	⬆️
... and 8 more

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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