parser: correctly identify error location on soft-keywords

[isidentical]

Currently it only throws the error on the first-seen node (?):

(.venv) (3.9.5) [  6:53ÖS ]  [ isidentical@msi:~ ]
 $ cat t.py                                                                                                   140ms
def something():
    match something:
        case t:
            pass

    match arg:
        case t:
            print(t)
        case z := u:
            print(a)
(.venv) (3.9.5) [  6:53ÖS ]  [ isidentical@msi:~ ]
 $ black -tpy310 --fast t.py                                                                                    2ms
error: cannot format t.py: Cannot parse: 2:10:     match something:
Oh no! 💥 💔 💥
1 file failed to reformat.
(.venv) (3.9.5) [  6:53ÖS ]  [ isidentical@msi:~ ]
 $ .venv310/bin/python -m ast t.py                                                                            147ms
Traceback (most recent call last):
 File "t.py", line 9
    case z := u:
           ^^
SyntaxError: expected ':'

[JelleZijlstra]

This is because it tries multiple grammars in parsing.py. I added some print statements:

(black) jelle@mbpt-root black % black -t py310 t.py
try <blib2to3.pgen2.grammar.Grammar object at 0x103180d30>
Cannot parse: 9:15:         case z := u:
try <blib2to3.pgen2.grammar.Grammar object at 0x103180cd0>
Cannot parse: 2:10:     match something:
error: cannot format t.py: Cannot parse: 2:10:     match something:

I think it shouldn't try other grammars if we're in -t py310 mode.

[isidentical]

Ah nice catch @JelleZijlstra, I thought it was a bug on the parser.