Make len-as-condition only fire when a len(x) call is made without an explicit comparison

[PaulRenvoise]

Steps

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• Add a ChangeLog entry describing what your PR does.
• If it's a new feature or an important bug fix, add a What's New entry in doc/whatsnew/<current release.rst>.
• Write a good description on what the PR does.

Description

This PR reduce the scope of len-as-condition to only care when a len(SEQUENCE) call is made without an explicit comparison. As stated in PEP8:

For sequences, (strings, lists, tuples), use the fact that empty sequences are false.

It also changes the checker's message and description to:

Do not use `len(SEQUENCE)` without comparison to determine if a sequence is empty

Used when Pylint detects that len(sequence) is being used without explicit comparison inside a condition to determine if a sequence is empty. Instead of coercing the length to a boolean, either rely on the fact that empty sequences are false or compare the length to a scalar.

Type of Changes

	Type
✓	🐛 Bug fix
✓	🔨 Refactoring

Related Issue

Closes #2684

[coveralls]

Coverage decreased (-0.02%) to 89.713% when pulling b2cd28a on PaulRenvoise:fix/len_as_condition into 095928c on PyCQA:master.

[PCManticore]

Thanks a lot for your PR @PaulRenvoise ! It was super clean and straightforward. 🥇