Awaitable _CoroGuard and fix ClientSession.close warning message

[litwisha]

What do these changes do?

1. _CoroGuard. is awaitable.
2. Fix warning message for ClientSession.close() method. (from ClientSession.close() is not coroutine to ClientSession.close() is coroutine)

Related issue number

#2026

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.

[asvetlov]

Warning text should be fixed.
The guard encourages await session.close() usage while we should support just session.close() for a while for sake of backward compatibility.

[codecov-io]

Codecov Report

Merging #2027 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2027      +/-   ##
==========================================
+ Coverage   97.05%   97.05%   +<.01%     
==========================================
  Files          38       38              
  Lines        7700     7704       +4     
  Branches     1347     1348       +1     
==========================================
+ Hits         7473     7477       +4     
  Misses        103      103              
  Partials      124      124

Impacted Files	Coverage Δ
aiohttp/helpers.py	96.9% <100%> (+0.02%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0957977...8a9c76f. Read the comment docs.

[asvetlov]

Please update warning message as well

[asvetlov]

Also please add a record in ./changes folder

[litwisha]

@asvetlov I have completed all the notes. It seems to be ready for merge

[asvetlov]

Thank you.
I'll process the PR a bit later.

[asvetlov]

Thanks!

[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.