The DFA for the rule

with_stmt: 'with' ( with_item (',' with_item)* | '(' with_item (',' with_item)* [','] ')' ) ':' suite

is:

DFA for with_stmt                                                                                                                                 [512/2103]
  State 0
    'with' -> 1
  State 1
    '(' -> 2
    with_item -> 3
  State 2
    with_item -> 4
  State 3
    ',' -> 5
    ':' -> 6
  State 4
    ')' -> 7
    ',' -> 8
  State 5
    with_item -> 3
  State 6
    suite -> 10
  State 7
    ':' -> 6
  State 8
    ')' -> 7
    with_item -> 4
  State 9
    ',' -> 5
    ':' -> 6
  State 10 (final)
  State 11
    ')' -> 7
    ',' -> 8

It works because the transition from State 1 into a "(" is going to prioritize the path:

0 -> 1 -> "(" -> 2

instead if 

0 -> 1 -> with_item -> 3