Fix readuntil work

[Tingaev]

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What do these changes do?

Fix work readuntil() with separator length more one character

Are there changes in behavior for the user?

No

Related issue number

Fixes #6701

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

Merging #6810 (fb0e463) into master (1f97c55) will decrease coverage by 0.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6810      +/-   ##
==========================================
- Coverage   97.50%   97.45%   -0.05%     
==========================================
  Files         103      103              
  Lines       29990    29999       +9     
  Branches     3643     3651       +8     
==========================================
- Hits        29243    29237       -6     
- Misses        567      581      +14     
- Partials      180      181       +1     

Flag	Coverage Δ
CI-GHA	97.36% <100.00%> (-0.05%)	:arrow_down:
OS-Linux	96.99% <100.00%> (-0.06%)	:arrow_down:
OS-Windows	95.39% <100.00%> (+<0.01%)	:arrow_up:
OS-macOS	96.69% <100.00%> (+<0.01%)	:arrow_up:
Py-3.10.8	97.16% <100.00%> (-0.06%)	:arrow_down:
Py-3.11.0	96.56% <100.00%> (+<0.01%)	:arrow_up:
Py-3.7.15	96.89% <100.00%> (+<0.01%)	:arrow_up:
Py-3.7.9	95.28% <100.00%> (+<0.01%)	:arrow_up:
Py-3.8.10	?
Py-3.8.14	96.78% <100.00%> (+<0.01%)	:arrow_up:
Py-3.9.13	95.17% <100.00%> (+<0.01%)	:arrow_up:
Py-3.9.14	96.58% <100.00%> (+<0.01%)	:arrow_up:
Py-3.9.15	96.76% <100.00%> (+<0.01%)	:arrow_up:
Py-pypy7.3.9	?
VM-macos	96.69% <100.00%> (+<0.01%)	:arrow_up:
VM-ubuntu	96.99% <100.00%> (-0.06%)	:arrow_down:
VM-windows	95.39% <100.00%> (+<0.01%)	:arrow_up:

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

Impacted Files	Coverage Δ
aiohttp/streams.py	97.45% <100.00%> (ø)
tests/test_streams.py	100.00% <100.00%> (ø)
tests/autobahn/client/client.py	46.42% <0.00%> (-50.00%)	:arrow_down:
tests/autobahn/test_autobahn.py	98.33% <0.00%> (-1.67%)	:arrow_down:

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more

[Tingaev]

@Dreamsorcerer ping

[patchback[bot]]

Backport to 3.9: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.9/ec714c820fd12aebe8d5d8594cd50345164d6bb0/pr-6810

Backported as https://github.com/aio-libs/aiohttp/pull/7079

🤖 @patchback I'm built with octomachinery and my source is open — https://github.com/sanitizers/patchback-github-app.