Improve Suggestion for empty TupleType syntax error (#9670)
* Improve Suggestion for empty TupleType syntax error
The mypy syntax for a function or method that takes zero parameters
is `() -> …`. Some new mypy users will reason by symmetry that the
syntax for a method that returns nothing is likely `(…) -> ()`.
A user who incorrectly annotates a function with `… -> ()` will be
given the suggestion `Use Tuple[T1, ..., Tn] instead of (T1, ..., Tn)`.
This suggestion is unlikely to help correct the user's misconception
about how to annotate the return type of a method that does not
explicitly return a value.
This PR adds a case to TupleType syntax error handling that returns a
helpful suggestion in the case where the tuple contains zero items.
Note: The error message casing in TupleType has grown large enough
that it likely warrants relocation to `MessageBuilder`, but as there
is not a `MessageBuilder` in accessible from `TypeAnalyzer` I have
decided to add this single error case the easy way. There is a
preexisting comment about the inaccessibility of `MessageBuilder`
in `RypeAnalyzer`'s `cannot_resolve_type method`.
I have added a test to `check-functions.test` that verifies the new
suggestion is printed when `-> ()` is used as a return type
annotation. I have also tested that a valid return type of
`-> Tuple[()]` remains without error.
* Clarify suggestion message
