Drop setup.py test support

[asvetlov]

Closes #3518

We decided to not support setup.py test facility anymore.
Use make test now.
@webknjaz has a plan to add tox configuration -- it would be a nice addition.
tox can be used by CI and users who comfortable with this workflow.
For now on my daily aiohttp hacking I personally prefer old good makefiles

[codecov-io]

Codecov Report

Merging #3752 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3752   +/-   ##
=======================================
  Coverage   97.87%   97.87%           
=======================================
  Files          43       43           
  Lines        8561     8561           
  Branches     1374     1374           
=======================================
  Hits         8379     8379           
  Misses         77       77           
  Partials      105      105

---

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[webknjaz]

I'd also add that setup.py test is officially discouraged during the last several years anyway.

[patchback]

Backport to 3.8: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 12b8e8c on top of patchback/backports/3.8/12b8e8c036df3b7e5ae2b7cff91f20b7c760ee62/pr-3752

Backporting merged PR #3752 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/3.8/12b8e8c036df3b7e5ae2b7cff91f20b7c760ee62/pr-3752 upstream/3.8

4. Now, cherry-pick PR Drop setup.py test support #3752 contents into that branch:

   $ git cherry-pick -x 12b8e8c036df3b7e5ae2b7cff91f20b7c760ee62

   If it'll yell at you with something like fatal: Commit 12b8e8c036df3b7e5ae2b7cff91f20b7c760ee62 is a merge but no -m option was given., add -m 1 as follows intead:

   $ git cherry-pick -m1 -x 12b8e8c036df3b7e5ae2b7cff91f20b7c760ee62

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Drop setup.py test support #3752 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.8/12b8e8c036df3b7e5ae2b7cff91f20b7c760ee62/pr-3752

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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