Add runner.serve_forever()

[vsmaxim]

What do these changes do?

This is a continuation of work from #4378. I've dropped cleanup call on serve_forever() task cancellation and updated the documentation accordingly.

Are there changes in behavior for the user?

No.

Related issue number

#2746

Original attempt from asvetlov: https://github.com/aio-libs/aiohttp/compare/master...server_forever

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [x] Documentation reflects the changes
• [x] 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.
• [x] Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.28571% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 97.36%. Comparing base (4601183) to head (b05fcaa). Report is 480 commits behind head on master.

Files	Patch %	Lines
aiohttp/web_runner.py	83.33%	1 Missing and 1 partial :warning:

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #7688    +/-   ##
========================================
  Coverage   97.36%   97.36%            
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  Files         106      106            
  Lines       31541    31661   +120     
  Branches     3593     3625    +32     
========================================
+ Hits        30709    30826   +117     
- Misses        630      631     +1     
- Partials      202      204     +2     

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[Dreamsorcerer]

Sorry for taking so long to look at this. I have a few questions about the implementation and expected behaviour:

• The original attempt ran cleanup on cancellation. Should this still happen? Seems it would match the asyncio.Server behaviour which says "Cancellation of serve_forever task causes the server to be closed."
  • I see @asvetlov suggested we do an async context manager for this one instead, so I guess the current version is correct. Will need a second PR to add the context manager though.
• The original also checks if there's any sites, should we include that check?
• Along those same lines, should we call Site.start() for each site, so the user doesn't need to call that themselves? This would match the asyncio.Server behaviour which calls Server.start_serving() from within Server.serve_forever().
• Should an explicit call to Runner.cleanup() cancel the future in serve_forever()? This would match asyncio.Server.close() which cancels the future.