Ignore class methods signatures with similarities "ignore-signatures" option enabled

[mhumetskyi]

Is your feature request related to a problem? Please describe

Initial already closed issue #3619.
One more related closed question with examples #4350.

Similarities "ignore-signatures" option currently only ignores functions' signatures.
Class methods won't be ignored with the current implementation which would be very useful to avoid false-negative duplicate-code errors for classes.

Describe the solution you'd like

With similarities "ignore-signatures" option enabled, class methods' signatures are ignored the same way as functions'.

[mhumetskyi]

@Pierre-Sassoulas hi again!
I have opened PR with the proposed solution.

While worked on that I have noticed that function tolineno property may not work correctly, but there is already opened issue for astroid: pylint-dev/astroid#868

We rely on tolineno property only when a function doesn't have a body (== when it has only docstring), so the impact is small.

It's just FYI in case somebody will report an issue that pylint doesn't work well in this case 🙃