gh-154359: Optimize UserList type checks to use list directly

[pranavchoudhary-tech]

This PR optimizes the type checking within UserList inside the collections module.

Since self.data is hardcoded to be an empty list [] at the beginning of UserList.__init__, the expression type(self.data) will always evaluate to the list type. We can replace type(initlist) == type(self.data) with a direct and more idiomatic type(initlist) is list identity check, bypassing the unnecessary type() call and attribute lookup while avoiding the slight anti-pattern of using the == operator to compare singleton type objects. The same optimization applies to the __add__, __radd__, and __iadd__ magic methods.

I ran the test_collections suite and verified that everything passes successfully with these changes.

Fixes #154359

• Issue: Optimize UserList type checks to use list directly #154359

[pranavchoudhary-tech]

Could a maintainer please apply the skip news label when they get a chance? This is just a minor internal optimization so I don't think a news entry is required.

[picnixz]

Performance improvements must always be backed by benchmarks. I don't see value in this proposal for a class that is not used a lot and where this change could have broken existing code elsewhere (the attribute is still public).