Forbid synchronous context managers

[asvetlov]

for ClientSession and test client/server.

Fixes #2540

[pfreixes]

LGTM

[asvetlov]

Implementation note: I use raise RuntimeError instead of just removing __enter__ methods for better exception text.
Figured out that TypeError is even better exception type.

[codecov-io]

Codecov Report

Merging #2543 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2543      +/-   ##
==========================================
- Coverage   97.18%   97.16%   -0.02%     
==========================================
  Files          40       40              
  Lines        8199     8193       -6     
  Branches     1441     1441              
==========================================
- Hits         7968     7961       -7     
- Misses         99      100       +1     
  Partials      132      132

Impacted Files	Coverage Δ
aiohttp/test_utils.py	98.21% <100%> (-0.03%)	⬇️
aiohttp/helpers.py	97.78% <0%> (-0.25%)	⬇️
tests/autobahn/client.py	95.06% <0%> (-0.03%)	⬇️

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[asvetlov]

@pfreixes please next time approve by github UI, not onlt by LGTM comment.
Sure, I have administrator privilege for skipping the check but very appreciate the following formal procedure.

[pfreixes]

Oki doki, one question can I click merge button but taking into account that must pass the CI pipeline to finally do it? Something like "Merge if CI passes"

[asvetlov]

IMHO Merge button becomes enabled only if CI passed.
There is no "delayed merge".

[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