import test.test_idle as ti
import unittest as u
u.main(ti)

has one failure:
Traceback (most recent call last):
  File "C:\Programs\Python39\lib\idlelib\idle_test\test_autocomplete.py", line 230, in test_fetch_completions
    if __main__.__file__ != ac.__file__:
AttributeError: module '__main__' has no attribute '__file__'

ac = autocomplete. The condition is followed by 
            self.assertNotIn('AutoComplete', small)  # See issue 36405

which is false when ac is run as main to run the test.  Adding "hasattr(__main__, '__file__') and " to the beginning of the condition should be sufficient.  I should add a note to #36405 explaining this conditional assertion better.

This is currently the only occurrence of __main__.__file__ in idlelib.

I was able to reproduce the reported issue and the suggested fix worked fine. Is there anything else missing like documentation note?

I would like to submit a PR for this @terry.reedy

Go ahead. Since the failure was user visible, there should be a note.  For IDLE issues, idlelib/NEWS.txt also needs an addition, but not until ready to merge.  So add a trial blurb or not as you wish.  In any case, I need to fix the 3.8 version and perhaps the 3.7 version before another IDLE commit.

I added a news entry to idlelib/NEWS.txt but don't know, where to add an additional note. The documentation of IDLE doesn't seem to be the right place. Should a note be added to Misc/NEWS.d/next or would it be redundant, because it is already added to idlelib/NEWS.txt?

Sorry, if I misunderstood you, but it is my first contribution to IDLE. Thanks for the guidance, I really appreciate it!

New changeset 905b3cd05f8d2c29e1605d109900e3e9d07af4d3 by Florian Dahlitz in branch 'master':
bpo-40723: Make IDLE autocomplete test run without __main__.__file__ (GH-20311)
https://github.com/python/cpython/commit/905b3cd05f8d2c29e1605d109900e3e9d07af4d3

New changeset a64df485a4c3ebb0caa1d62c02246cd43d0e976e by Miss Islington (bot) in branch '3.8':
bpo-40723: Make IDLE autocomplete test run without __main__.__file__ (GH-20311)
https://github.com/python/cpython/commit/a64df485a4c3ebb0caa1d62c02246cd43d0e976e

New changeset 82397e2d97f89fdf36cb8eaf3b2d7c407456ec78 by Miss Islington (bot) in branch '3.7':
bpo-40723: Make IDLE autocomplete test run without __main__.__file__ (GH-20311)
https://github.com/python/cpython/commit/82397e2d97f89fdf36cb8eaf3b2d7c407456ec78

New changeset 874506cff9aff2cb5eb0192c878e08ded18326a9 by Miss Islington (bot) in branch '3.9':
bpo-40723: Make IDLE autocomplete test run without __main__.__file__ (GH-20311)
https://github.com/python/cpython/commit/874506cff9aff2cb5eb0192c878e08ded18326a9