remove check_python_3 (broken with click>=8.1.0)

[graingert]

fixes #6013

• check_python_3 is redundant on Python 3.7+ and distributed only supports 3.8+
• _unicodefun was removed on click 8.1.0 which causes check_python_3 to fail

• Closes ImportError: cannot import name '_unicodefun' from 'click' #6013
• Tests added / passed
• Passes pre-commit run --all-files

[github-actions]

Unit Test Results

       12 files  +         7         12 suites  +7   6h 4m 10s ⏱️ + 4h 53m 12s
  2 674 tests +  1 472    2 592 ✔️ +  1 434    81 💤 +  46  1 ❌  -   8 
15 948 runs  +12 357  15 086 ✔️ +11 649  861 💤 +726  1 ❌  - 18 

For more details on these failures, see this check.

Results for commit 00d81dc. ± Comparison against base commit fb78273.

♻️ This comment has been updated with latest results.

[fjetter]

Something is broken in the conda build that looks related. There is a traceback that go cannot be imported

[mrocklin]

Thanks all for handling this quickly

[jakirkham]

@graingert do we need to require click 8.1.0+ after this change or will this still work with earlier versions of click?

[graingert]

@graingert do we need to require click 8.1.0+ after this change or will this still work with earlier versions of click?

generally this will still work with earlier versions of click. The code that I removed in the PR was trying to replace a hard failure that was a regular occurrence on python 3.6 and "very rare" on python 3.7+ with a warning. This "very rare" hard failure case is now possible again when running on an old click and a new distributed.

[jakirkham]

Thanks for clarifying. How old of a version of click is still stable? Perhaps that should determine the lower bound