Add fix to remove unused standard library imports from __init__.py files

[zanieb]

See #10365 (comment)

We currently do not offer any fixes for unused imports in __init__.py files.

This issue is to fix standard library imports by removing them from the file. We should probably label this fix as unsafe.

[plredmond]

For the purposes of this check, I'm going to consider from __future__ import ... style imports to be "standard library".

[charliermarsh]

I think from __future__ imports can just be ignored entirely, since they should never be removed as "unused".

[plredmond]

Considered together with #10391, does this summary of behaviors seem right?

Old table

for some unused import... 

future                            → no diagnostic or fix
stdlib ∧  in_init                 → unsafe, remove
stdlib ∧ ¬in_init                 → safe,   remove
1stpty ∧  in_init                 → safe,   move to __all__ or convert to explicit-import
1stpty ∧ ¬in_init                 → safe,   remove
3rdpty ∧  in_init ∧  ignore_init  → no diagnostic or fix
3rdpty ∧  in_init ∧ ¬ignore_init  → unsafe, remove
3rdpty ∧ ¬in_init                 → safe,   remove

[charliermarsh]

My impression was that for 3rdpty ∧ in_init, we never offered a fix.

[zanieb]

That summary looks reasonable to me, with the addition that we should probably deprecate and remove the ignore_init option and remove third-party imports by default?

[charliermarsh]

Yeah I revise my comment -- what Zanie says sounds right.

[plredmond]

Ok, I've collapsed the table above. Here's an edited version without the ignore_init bit. I'll implement this, and try to make it flexible enough that I can PR #10390 separately from #10391 and so that we can more easily make changes to it in the future if desired.

for some unused import... 

future             → no diagnostic or fix
stdlib ∧  in_init  → unsafe, remove
stdlib ∧ ¬in_init  → safe,   remove
1stpty ∧  in_init  → safe,   move to __all__ or convert to explicit-import
1stpty ∧ ¬in_init  → safe,   remove
3rdpty ∧  in_init  → unsafe, remove
3rdpty ∧ ¬in_init  → safe,   remove