Use gdbpy_enter in py-breakpoint.c

A few spots in py-breakpoint.c acquire the GIL manually.  However,
because these spots generate events, and because events are expected
to be arbitrary gdb-flavored Python code, it's important to use
gdbpy_enter instead, in order to ensure that the other gdb-related
Python globals are set correctly.

This patch makes this change.  Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-06-10  Tom Tromey  <tom@tromey.com>

	* python/py-breakpoint.c (gdbpy_breakpoint_created)
	(gdbpy_breakpoint_deleted, gdbpy_breakpoint_modified): Use
	gdbpy_enter.