Order of overloads of index method can cause failure to type-check when a type is a sequence of itself (example: str) #100

First off, thank you for developing and maintaining this library! It's excellent and very useful. I have a minor bug to report.

Consider the following code:

from ordered_set import OrderedSet

oset: OrderedSet[str] = OrderedSet(["apple", "banana", "pear"])
apple_index: int = oset.index("apple")
print(oset[apple_index+1])

This should work—and it does! However, running mypy on it fails with the following error:

bug.py:4: error: Incompatible types in assignment (expression has type "list[int]", variable has type "int")  [assignment]

Why? Consider how the index method is defined:

@overload
def index(self, key: Sequence[T]) -> List[int]:
    ...

@overload
def index(self, key: T) -> int:
    ...

mypy sees we are calling index with a str and...this matches the first overload, as str is a Sequence[str]. mypy then concludes that the result is a list[int], causing the error.

The solution is to re-order the overloads so mypy sees, and uses, what is currently the second one. The actual implementation of index already does the right thing for strs.