gdb/python: move PyLong_From* calls into py-utils.c

gdb/python: move PyLong_From* calls into py-utils.c

We already have two helper functions in py-utils.c:

  gdb_py_object_from_longest (LONGEST l)
  gdb_py_object_from_ulongest (ULONGEST l)

these wrap around calls to either PyLong_FromLongLong,
PyLong_FromLong, or PyInt_From_Long (if Python 2 is being used).

There is one place in gdb/python/* where a call to PyLong_FromLong was
added outside of the above utility functions, this was done in the
recent commit:

  commit 55789354fcbaf879f3ca8475b647b2747dec486e
  Date:   Fri May 14 11:56:31 2021 +0200

      gdb/python: add a 'connection_num' attribute to Inferior objects

In this commit I replace the direct use of PyLong_FromLong with a call
to gdb_py_object_from_longest.  The only real change with this commit,
is that, for Python 2, we will now end up calling PyInt_FromLong
instead of PyLong_FromLong, but this should be invisible to the user.
For Python 3 there should be absolutely no change.

gdb/ChangeLog:

	* python/py-inferior.c (infpy_get_connection_num): Call
	gdb_py_object_from_longest instead of PyLong_FromLong directly.