Fix IndexError in strip_whitespace for collapsed parenthesis groups

[ekanshul]

Fixes #885.

sqlparse.format("( AS )", strip_whitespace=True) raised IndexError because grouping collapses that input into a Parenthesis with a single nested Identifier. _stripws_parenthesis then indexed tokens[1] and tokens[-2], which do not exist.

This change skips the inner/outer whitespace pops when the group has fewer than three tokens and falls through to the default strip. Well-formed groups such as ( 1 = 2 ) are unchanged.

>>> import sqlparse
>>> sqlparse.format("( AS )", strip_whitespace=True)
'( AS )'
>>> sqlparse.format("select * from foo where ( 1 = 2 )", strip_whitespace=True)
'select * from foo where (1 = 2)'

Added a regression test. Full suite: 507 passed, 2 xfailed, 1 xpassed.