[backport][3.7][PR#5498] Fix how pure-Python HTTP parser interprets //

[webknjaz]

(cherry picked from commit f2afa2f)

What do these changes do?

Are there changes in behavior for the user?

This makes the HTTP queries with a leading double slash to
be parsed as a query-path and not a hostname by the
pure-Python HTTP parser.

Related issue number

N/A

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 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]

Codecov Report

Merging #5499 (dc2dfa7) into 3.7 (5c1efbc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              3.7    #5499   +/-   ##
=======================================
  Coverage   97.55%   97.55%           
=======================================
  Files          44       44           
  Lines        8818     8820    +2     
  Branches     1419     1419           
=======================================
+ Hits         8602     8604    +2     
  Misses        101      101           
  Partials      115      115           

Flag	Coverage Δ
unit	97.39% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiohttp/http_parser.py	97.52% <100.00%> (+0.01%)	⬆️

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