Cleanup after dropping Python 3.6

[Dreamsorcerer]

FYI, to find these, I skim through the results of:
fgrep -Rn -e '36' -e '3.6' -e '3,6' -e '3, 6'
fgrep -Rn -e '37' -e '3.7' -e '3,7' -e '3, 7'

[Dreamsorcerer]

Appears multidict is not pinned in linter stage and now causing some type errors. Will fix in a separate PR tomorrow.

[codecov]

Codecov Report

Merging #6549 (50fef93) into master (be4893b) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6549      +/-   ##
==========================================
+ Coverage   93.34%   93.36%   +0.02%     
==========================================
  Files         104      104              
  Lines       30633    30624       -9     
  Branches     3083     3080       -3     
==========================================
- Hits        28594    28592       -2     
+ Misses       1863     1859       -4     
+ Partials      176      173       -3     

Flag	Coverage Δ
unit	93.28% <88.88%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
aiohttp/connector.py	93.93% <ø> (ø)
aiohttp/multipart.py	96.12% <ø> (ø)
aiohttp/streams.py	97.45% <ø> (ø)
aiohttp/helpers.py	95.76% <100.00%> (ø)
aiohttp/web_fileresponse.py	97.91% <100.00%> (-0.02%)	⬇️
tests/conftest.py	68.42% <100.00%> (+1.75%)	⬆️
tests/test_client_exceptions.py	100.00% <100.00%> (+2.35%)	⬆️

---

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[patchback]

Backport to 3.9: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 1dbfbb4 on top of patchback/backports/3.9/1dbfbb433f3f7cd05ca97288805dec9e296d75de/pr-6549

Backporting merged PR #6549 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/aio-libs/aiohttp.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.9/1dbfbb433f3f7cd05ca97288805dec9e296d75de/pr-6549 upstream/3.9

4. Now, cherry-pick PR Cleanup after dropping Python 3.6 #6549 contents into that branch:

   $ git cherry-pick -x 1dbfbb433f3f7cd05ca97288805dec9e296d75de

   If it'll yell at you with something like fatal: Commit 1dbfbb433f3f7cd05ca97288805dec9e296d75de is a merge but no -m option was given., add -m 1 as follows intead:

   $ git cherry-pick -m1 -x 1dbfbb433f3f7cd05ca97288805dec9e296d75de

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Cleanup after dropping Python 3.6 #6549 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.9/1dbfbb433f3f7cd05ca97288805dec9e296d75de/pr-6549

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
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my source is open — https://github.com/sanitizers/patchback-github-app.