Input prompt stripping broken with multiline data structures

[fperez]

We now strip input prompts always, but the logic seems to be broken in certain cases, as reported by @matthew-brett on-list. Some cases that work:

In [3]: >>> x =1

In [4]: x
Out[4]: 1

In [5]: >>> if x:
   ...:     ...     print 1
   ...:     ...     
1

But multilie data structures throw an error:

In [2]: >>> a = {'X':1,
   ...: ... 'U':2}
  File "", line 2
    ... 'U':2}
    ^
SyntaxError: invalid syntax

In [6]: >>> a = [1,2,
   ...: ... 3]
  File "", line 2
    ... 3]
    ^
SyntaxError: invalid syntax

In [9]: >>> a = (1,2,
   ...: ... 3)
  File "", line 2
    ... 3)
    ^
SyntaxError: invalid syntax

[fperez]

#877 definitely improves things a lot. There's an edge case left standing, as indicated there:

In [3]: >>> a = [1
   ...: ... ,2]
  File "<ipython-input-3-5b39dccea2ae>", line 2
    ... ,2]
     ^
SyntaxError: invalid syntax

which happens to be valid in python.

But I'm lowering the priority b/c with this fix, most real-world cases are covered.

[minrk]

fixed by #2447