Use gdbpy_enter in python_interactive_command

Use gdbpy_enter in python_interactive_command

This changes python_interactive_command to use gdbpy_enter.
Previously this function was leaving a dangling cleanup -- this is
sort of ok in a command function, but IMO it's still better to clean
up.

2017-01-10  Tom Tromey  <tom@tromey.com>

	* python/python.c (python_interactive_command): Use gdbpy_enter.