Fixes and closes #129545

Overview:

The SyntaxError: parameter without a default follows parameter with a default error message has been modified to SyntaxError: positional parameter without a default follows parameter with a default to clarify error messages.

As mentioned in #129545

@smheidrich

Trying to define a function in which a positional parameter follows a parameter with a default value, we get a SyntaxError as expected:

>>> def f(a=1, b):
 File "<python-input-0>", line 1
 def f(a=1, b):
            ^
SyntaxError: parameter without a default follows parameter with a default

But the message that comes with it is not as accurate as it could be, as we can easily construct a valid function signature in which a parameter without a default follows a parameter with a default so long as both are keyword-only parameters:

>>> def f(*, a=1, b):
...     pass
...     
>>> 

(I didn't use a=1, *, b for this counterexample because then one could argue that, if "follows" is interpreted as "follows immediately", we did resolve the situation described in the error message, making it formally accurate, albeit confusing.)

As also mentioned by @smheidrich:

It's not just a matter of accuracy for accuracy's sake, however: This change would also hint to people who don't know or have forgotten about keyword-only parameters that there is another possibility of resolving the issue than the ones implied by the current message.

The new error message, should be accurate for most cases and should not be confusing. However, if there are cases where the original error message might be more appropriate ex when dealing with keyword-only parameters or other specific syntax rules but the new change would cause minimal confusion compared to the problem that this pr intends to solve.

• Issue: SyntaxError: parameter without a default follows parameter with a default is inaccurate #129545