Add typing to checker and plugin attributes of PyLinter

[DanielNoord]

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Type of Changes

	Type
✓	🔨 Refactoring

Description

Seems like we made an error in #5004 that I didn't catch in my review.

Checkers __implements__ Ichecker, but this isn't picked up on by mypy. The typing should thus be BaseChecker.

[coveralls]

Pull Request Test Coverage Report for Build 1606600609

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.691%

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Totals
Change from base Build 1605912068:	0.0%
Covered Lines:	14300
Relevant Lines:	15263

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💛 - Coveralls

[Pierre-Sassoulas]

I think I've already done that in #5558 😄 ? (Well I used an union of both sometime)

[DanielNoord]

I didn't check that, but if you did we should remove IChecker. mypy has no way to interact with it so it doesn't really provide us with much..

[Pierre-Sassoulas]

See https://github.com/PyCQA/pylint/pull/5558/files#diff-f7184ace78676137bed7c4c7efec25dca1aa06ed21166e31396ac450f78400c0R25 (the whole file) and https://github.com/PyCQA/pylint/pull/5558/files#diff-3a681c16a79ae9818f939e08df8b9bb92ef9ce6d5fc65c360d7c9b526c3c3410R724

I think it makes sense to first merge this MR, as IChecker does not do much, then rebase #5558 on it ?

[DanielNoord]

Sounds good!