bpo-20490: Improve circular import error messaging

[asottile]

https://bugs.python.org/issue20490

[asottile]

This is largely based on the work in https://bugs.python.org/issue33237 by @serhiy-storchaka #6398

[asottile]

demo

$ tail -n999 a/*.py
==> a/b.py <==
from a.c import d

e = 1

==> a/c.py <==
from a.b import e

d = 1

==> a/__init__.py <==

before

$ python3.8 -c 'import a.b'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/tmp/x/a/b.py", line 1, in <module>
    from a.c import d
  File "/tmp/x/a/c.py", line 1, in <module>
    from a.b import e
ImportError: cannot import name 'e' from 'a.b' (/tmp/x/a/b.py)

after

$ ~/workspace/cpython/python -c 'import a.b'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/tmp/x/a/b.py", line 1, in <module>
    from a.c import d
  File "/tmp/x/a/c.py", line 1, in <module>
    from a.b import e
ImportError: cannot import name 'e' from 'a.b' (partially initialized module -- most likely due to a circular import) (/tmp/x/a/b.py)

[cool-RR]

Thanks for working on this @asottile ! If this old idea will come true, that'd be cool.

[asottile]

(I'm rebasing to presumably fix the CI failure -- it doesn't seem related to my change, but I've been surprised before by macos 😆)

[miss-islington]

Thanks @asottile for the PR, and @zooba for merging it 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[miss-islington]

Sorry @asottile and @zooba, I had trouble checking out the 3.8 backport branch.
Please backport using cherry_picker on command line.
cherry_picker 65366bc8bdc4716ebc361e622590b45a6e5aef07 3.8

[bedevere-bot]

GH-15791 is a backport of this pull request to the 3.8 branch.

[pablogsal]

This pull request introduced a reference leak:

https://buildbot.python.org/all/#/builders/1/builds/710

This is being tracked in:

https://bugs.python.org/issue38090