Mark imports in __all__ as used

[kreathon]

At the moment, symbols that are referenced in string form in the __all__ variable assignment are not marked as used.

Example

# Currently, 'Bar' would be marked as unused.
from foo import Bar

__all__ = ["Bar"]

Implementation

Add visit_Assign to cover assignments of the following form:

# Default case
__all__ = ["A"]

# Tuples are also tolerated
__all__ = ("B", )

Limitations

The implementation cannot cover dynamic definitions e.g.:

a = ["A"]
__all__ = a

Related Issue

This PR is addressing the issue #172.

Checklist:

• I have updated the documentation in the README.md file or my changes don't require an update.
• I have added an entry in CHANGELOG.md.
• I have added or adapted tests to cover my changes.
• I have run tox -e fix-style to format my code and checked the result with tox -e style.

[codecov-commenter]

Codecov Report

Merging #282 (118eae2) into master (34e93b0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #282   +/-   ##
=======================================
  Coverage   99.37%   99.38%           
=======================================
  Files          19       19           
  Lines         638      648   +10     
=======================================
+ Hits          634      644   +10     
  Misses          4        4           

Impacted Files	Coverage Δ
vulture/core.py	99.39% <100.00%> (+0.01%)	⬆️

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[jendrikseipp]

Code looks great, thanks! I only have one suggestion, see below. And I was wondering if we should add more tests to cover edge cases such as ignoring code like the following import my_module; my_module.__all__ = ["Foo"]. What do you think?

[kreathon]

Thank you for your feedback.

I am going to add a few more test cases (also to increase the branch coverage).

[jendrikseipp]

I agree about the readability, but let's go with that version anyway. Thanks for your great work!