Fix bug in fix_print.py fixer

[abjonnes]

When the print ends with a non-space whitespace character, an extra
space character should not be printed at the end.

From the python2 documentation:

A space is written before each object is (converted and) written, unless the output system believes it is positioned at the beginning of a line. This is the case (1) when no characters have yet been written to standard output, (2) when the last character written to standard output is a whitespace character except ' ', or (3) when the last write operation on standard output was not a print statement.

The changes here don't do this perfectly (e.g. when a printed string variable ends with a tab), but capture the common case of when a literal string which ends in a tab or line feed is printed with a trailing comma, like print "something\t",.

Addresses (at least partially) #473.

[edschofield]

Many thanks, @abjonnes !