[Bug 1883175] Re: missing support for python3.8 language features

[Ted Kern]

Prepared related SRUs to fix test failures in other packages caused by
this update:

https://bugs.launchpad.net/ubuntu/+source/budgie-extras/+bug/1907729
https://bugs.launchpad.net/ubuntu/+source/update-manager/+bug/1907727
https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1907728

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https://bugs.launchpad.net/bugs/1883175

Title:
  missing support for python3.8 language features

Status in pycodestyle package in Ubuntu:
  Fix Released
Status in pyflakes package in Ubuntu:
  Fix Released
Status in python-flake8 package in Ubuntu:
  Fix Released
Status in pycodestyle source package in Focal:
  Fix Committed
Status in pyflakes source package in Focal:
  Fix Committed
Status in python-flake8 source package in Focal:
  Fix Committed

Bug description:
  [Impact]

  Python 3.8 language features include some syntax that python3-flake8
  3.7.9-2 registers as an error.

  flake8 3.8.x is now available, released after Focal. The minor version
  of flake8 matches the supported python version, so a release from the
  3.8.x series should be ported to Focal.

  This version bump requires dependency pycodestyle >= 2.6.0, which is
  where the bugfix occurs. The dependency pyflakes must also be updated.

  [Test Case]

  Create a file test.py
  Use python3 language features, e.g.
  ```
  while (a := float(input())) < 10 :
      print(f'{a=} is too much!')
  print(f'{a=} works fine')
  ```
  Run `python3 -m flake8 test.py`

  Result:
  "pyflakes" failed during execution due to "'FlakesChecker' object has no attribute 'NAMEDEXPR'"
  Run flake8 with greater verbosity to see more details
  test.py:1:9: E203 whitespace before ':'
  test.py:1:10: E231 missing whitespace after ':'

  `echo $?` shows a return value of 1

  With the 3.8.3-1 version of the package proposed for Groovy
  (https://launchpad.net/ubuntu/+source/python-flake8/3.8.3-1) `test.py`
  produces no output and returns 0

  [Regression Potential]

  Potential is minimal as this is a minor version bump, which upstream
  defines as only offering additional, optional checks and not breaking
  backward compatibility. No reported breakages between 3.7.9 and 3.8.3
  are shown in upstream issue tracker.

  Any regressions would appear as unexpected errors emitted when running
  the linter on previously error-less, valid python code.

  Upstream QA:
  https://flake8.pycqa.org/en/latest/internal/releases.html#process
  outlines the required tests a release must pass, all of which
  completed in https://gitlab.com/pycqa/flake8/-/pipelines/154124695

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