fix Issue #1861: Client request timeout parameters are inconsistent

[ykshatroff]

What do these changes do?

Use sentinel (instead of None) for checking the timeout arguments passed.

aiohttp/client.py:

• ClientSession.init():
  • set the read_timeout argument default value to sentinel
  • if it is equal to sentinel during the call, set it to the value of DEFAULT_TIMEOUT
• ClientSession._request():
  • set the timeout argument default value to sentinel
  • if it is equal to sentinel during the call, set it to the value of self.read_timeout

Update documentation to match the consistent usage of timeouts.
Update tests (excluding the confusing timeout=None argument usage).

Are there changes in behavior for the user?

• The read_timeout argument to ClientSession constructor, when omitted, now defaults to DEFAULT_TIMEOUT which is 5 min, and not to None which is no timeout.
• The timeout argument should be omitted in session's request() methods in order to use the session's read_timeout.
• The timeout argument explicitly passed as None to session's request() methods now disables the request timeout instead of resulting in the session's read_timeout being used.

Related issue number

Issue #1861 Client request timeout parameters are inconsistent

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.
• Add a new entry to CHANGES.rst
  • Choose any open position to avoid merge conflicts with other PRs.
  • Add a link to the issue you are fixing (if any) using #issue_number format at the end of changelog message. Use Pull Request number if there are no issues for PR or PR covers the issue only partially.

[codecov-io]

Codecov Report

Merging #1862 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1862   +/-   ##
=======================================
  Coverage   97.19%   97.19%           
=======================================
  Files          37       37           
  Lines        7533     7533           
  Branches     1308     1308           
=======================================
  Hits         7322     7322           
  Misses         89       89           
  Partials      122      122

Impacted Files	Coverage Δ
aiohttp/client.py	97.09% <100%> (ø)	⬆️

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[fafhrd91]

thanks!

[lock]

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