builtins.slice: more precise __new__ overloads and defaults for StopT and StepT.

[randolf-scholz]

Alternative to #12904, letting step default to start|stop (if intersections are ever added, use start&stop)

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

mongo-python-driver (https://github.com/mongodb/mongo-python-driver)
- pymongo/synchronous/auth.py:192: error: Argument 1 to "getnameinfo" has incompatible type "tuple[str, int] | tuple[str, int, int, int] | tuple[int, bytes]"; expected "tuple[str, int] | tuple[str, int, int, int]"  [arg-type]
- pymongo/asynchronous/auth.py:195: error: Argument 1 to "getnameinfo" has incompatible type "tuple[str, int] | tuple[str, int, int, int] | tuple[int, bytes]"; expected "tuple[str, int] | tuple[str, int, int, int]"  [arg-type]

urllib3 (https://github.com/urllib3/urllib3)
- test/contrib/test_socks.py:120: error: Incompatible return value type (got "list[tuple[AddressFamily, SocketKind, int, str, tuple[str, int] | tuple[str, int, int, int] | tuple[int, bytes]]]", expected "list[tuple[AddressFamily, SocketKind, int, str, tuple[str, int] | tuple[str, int, int, int]]]")  [return-value]

trio (https://github.com/python-trio/trio)
- src/trio/_socket.py:215: error: Incompatible return value type (got "list[tuple[AddressFamily, SocketKind, int, str, Union[tuple[str, int], tuple[str, int, int, int], tuple[int, bytes]]]]", expected "list[tuple[AddressFamily, SocketKind, int, str, Union[tuple[str, int], tuple[str, int, int, int]]]]")  [return-value]
- src/trio/_socket.py:244: error: Argument 1 to "to_thread_run_sync" has incompatible type "Callable[[Union[bytes, str, None], Union[bytes, str, int, None], int, int, int, int], list[tuple[AddressFamily, SocketKind, int, str, Union[tuple[str, int], tuple[str, int, int, int], tuple[int, bytes]]]]]"; expected "Callable[..., list[tuple[AddressFamily, SocketKind, int, str, Union[tuple[str, int], tuple[str, int, int, int]]]]]"  [arg-type]
- src/trio/_tests/test_socket.py:130: error: Argument 1 to "MonkeypatchedGAI" has incompatible type "Callable[[Union[bytes, str, None], Union[bytes, str, int, None], int, int, int, int], list[tuple[AddressFamily, SocketKind, int, str, Union[tuple[str, int], tuple[str, int, int, int], tuple[int, bytes]]]]]"; expected "Callable[[Union[str, bytes, None], Union[str, bytes, int, None], int, int, int, int], list[tuple[AddressFamily, SocketKind, int, str, Union[tuple[str, int], tuple[str, int, int, int]]]]]"  [arg-type]

[srittau]

Let's start with this PR as it fixes a few primer hits.