Fix `Connection.connect()` return type

Fix `Connection.connect()` return type

Now `connect()` returns a `Connection[Tuple]`, whereas
`connect(row_factory=something)` return the type of what row factory
produces. The implementation of this is somewhat brittle, but that's
mypy for you: @dlax (thank you!) noticed that defining `**kwargs:
Union[str, int]` helped to disambiguate the row_factory param. I guess
we will make a best-effort to maintain this "interface". Everything is
to be documented.

Strangely, mypy cannot figure out the type of

    conn = await self.connection_class.connect(
        self.conninfo, **self.kwargs
    )

in the async pool, but it can for the sync one (without the `await`).
Added explicit type to disambiguate, on both the classes, for symmetry.

Added regression tests to verify that refactoring doesn't break type
inference.