Fix false positive when iterating over `Enum`

[anudaweerasinghe]

Description

• Creates dictionary enum_class_vars that maps subclasses of Enum to variables defined within them
• Updates visit_Call to add values in enum_class_vars to used_names if the function is list, tuple, or set and the argument is a subclass of Enum
• Implements visit_For to handle the case where we iterate over an Enum, i.e. for e in E

Related Issue

Resolves #304

Checklist:

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

[codecov-commenter]

Codecov Report

Merging #312 (8743ead) into main (e2e84d0) will increase coverage by 0.03%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main     #312      +/-   ##
==========================================
+ Coverage   98.94%   98.98%   +0.03%     
==========================================
  Files          21       21              
  Lines         665      689      +24     
==========================================
+ Hits          658      682      +24     
  Misses          7        7              

Impacted Files	Coverage Δ
vulture/core.py	98.63% <100.00%> (+0.09%)	:arrow_up:

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