[3.6] Fix python 3.8 warnings (#4264).

[TBBle]

(cherry picked from commit 159b039)

Conflict resolution based on #4265

Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>

Fixes: #4477

[TBBle]

CI (Lint docs) failure seems to be due to https://github.com/aio-libs/aiohttp/blob/3.6/requirements/doc.txt not including multidict. It's present in https://github.com/aio-libs/aiohttp/blob/3.6/requirements/ci-wheel.txt which is pulled by https://github.com/aio-libs/aiohttp/blob/3.6/requirements/ci.txt.

Deps-install definition from https://github.com/aio-libs/aiohttp/blob/3.6/.azure-pipelines/stage-lint.yml#L98:

    - script: |
        apt install libenchant-dev
        pip install -r requirements/doc-spelling.txt
        pip install -r requirements/towncrier.txt
      displayName: 'Install deps'

I see no different in 3.7, and I'm not sure why this hasn't failed in previous builds, as nothing relevant seems to have changed, and this change does nothing but remove code...

[TBBle]

On closer examination, the issue might not be the multidict exception, but the failure importing u from blockdiag.utils.compat. This is probably because (from Install Deps):

Collecting sphinxcontrib-blockdiag==1.5.5
  Downloading sphinxcontrib_blockdiag-1.5.5-py2.py3-none-any.whl (7.0 kB)
...
Collecting blockdiag>=1.5.0
  Downloading blockdiag-2.0.1-py3-none-any.whl (2.7 MB)

blockdiag 2.0.1 does not have u in the blockdiag.utils.compat module, but sphinxcontrib-blockdiag 1.5.5 was insufficiently-protected from this API change.

This suggests we'd need a backport of #4427/#4428 (for blockdiag/sphinxcontrib-blockdiag@3eb3e0f), or perhaps just put something in this branch's requirements list when we require sphinxcontrib-blockdiag to also require blockdiag < 2.0.0.

[TBBle]

I believe the cherry-pick is correct, but due to 3.6-branch bitrot, getting a valid CI run for this PR depends on #4576 or similar fixes for 3.6 first.