Pre-parser code must come after imports in the generated parser

[felixp7]

See https://gist.github.com/felixplesoianu/90d104fa8ad75b02155b -- if I try to run the generated parser from the command line, I get the following exception:

  File "exprcalc3.py", line 11
    from __future__ import print_function
SyntaxError: from __future__ imports must occur at the beginning of the file

Editing exprcalc3.py accordingly is a trivial workaround, but ideally it shouldn't be needed. Thank you.