C = TypeVar('C', bound='Copyable')
class Copyable(Protocol):
    def copy(self: C) -> C:
        ...

class One:
    def copy(self) -> 'One':
        ...

T = TypeVar('T', bound='Other')
class Other:
    def copy(self: T) -> T:
        ...

c: Copyable
c = One()  # OK
c = Other()  # Also OK