gh-125326: Improve SyntaxError for invalid type params definiton

[sobolevn]

This does not fix 100% of cases, but makes the situation better. What is not covered? For example:

>>> def some[call()](): ...
  File "<python-input-0>", line 1
    def some[call()](): ...
            ^
SyntaxError: expected '('

This happens because of how rules detect top-level type_params. I don't think that there's a clean way to handle this.

However, my patch also fixes a lot of possible corner cases.

• Issue: Improve SyntaxError for invalid type param definition #125326

[JelleZijlstra]

This looks correct but I'm not sure the special error messages are worth it; I haven't personally seen evidence from users who saw these errors and were confused. Adding more special-case error messages to the grammar increases the complexity of the system. I'll leave that to the parser maintainers to decide.

[JelleZijlstra]

Suggested change

	"Type parameter list cannot be empty")}
	"type parameter list cannot be empty")}

We're not completely consistent but most syntax error messages are lowercase.

[sobolevn]

Yeah, I kinda agree that this PR is not ideal, as I said in the first post.
Since it does not really cover all possible cases.