Require all overloads be either sync or async

[sobolevn]

Overloads are a typing-only contruct, so real python implementation is either sync or async. Having some items as async and some as sync does not make much sense and is an error probably.

Closes #16166

[sobolevn]

Please, see #14996
Sometimes you expect sync overloads when funtion returns AsyncIterable or AsyncGenerator

@hauntsaninja can you please explain this API, please? This seems like a bug to me :)

[github-actions]

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

discord.py (https://github.com/Rapptz/discord.py)
+ discord/client.py:1149: error: All overloaded items but be either sync or async  [misc]

psycopg (https://github.com/psycopg/psycopg)
+ psycopg/psycopg/_typeinfo.py:56: error: All overloaded items but be either sync or async  [misc]

jinja (https://github.com/pallets/jinja)
+ src/jinja2/filters.py:1475: error: All overloaded items but be either sync or async  [misc]

Expression (https://github.com/cognitedata/Expression)
+ expression/collections/asyncseq.py:75: error: All overloaded items but be either sync or async  [misc]

[hauntsaninja]

Yeah, see the documentation I added here https://mypy.readthedocs.io/en/latest/more_types.html#asynchronous-iterators in particular the last example with launch:

# The type of the overloads is independent of the implementation.
# In particular, their type is not affected by whether or not the
# implementation contains a `yield`.
# Use of `def`` makes it clear the type is Callable[..., AsyncIterator[int]],
# whereas with `async def` it would be Callable[..., Coroutine[Any, Any, AsyncIterator[int]]]
@overload
def launch(*, count: int = ...) -> AsyncIterator[int]: ...
@overload
def launch(*, time: float = ...) -> AsyncIterator[int]: ...

async def launch(*, count: int = 0, time: float = 0) -> AsyncIterator[int]:
    # The implementation of launch is an async generator and contains a yield
    yield 0

As I mention, the root cause is that the presence of yield inside a function body has an effect on the type of the function