Introduce gdbpy_enter_varobj and use it

Introduce gdbpy_enter_varobj and use it

This introduces gdbpy_enter_varobj, a subclass of gdbpy_enter; then
changes one function in py-varobj.c to use it.  gdbpy_enter_varobj
takes a varobj as an argument, similar to varobj_ensure_python_env.

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

	* varobj.c (gdbpy_enter_varobj): New constructor.
	* python/python-internal.h (gdbpy_enter_varobj): New class.
	* python/py-varobj.c (py_varobj_get_iterator): Use
	gdbpy_enter_varobj.