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

Define a 'connection_num' attribute for Inferior objects.  The
read-only attribute is the ID of the connection of an inferior, as
printed by "info inferiors".  In GDB's internal terminology, that's
the process stratum target of the inferior.  If the inferior has no
target connection, the attribute is None.

gdb/ChangeLog:
2021-05-14  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* python/py-inferior.c (infpy_get_connection_num): New function.
	(inferior_object_getset): Add a new element for 'connection_num'.
	* NEWS: Mention the 'connection_num' attribute of Inferior objects.

gdb/doc/ChangeLog:
2021-05-14  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* python.texi (Inferiors In Python): Mention the 'connection_num'
	attribute.

gdb/testsuite/ChangeLog:
2021-05-14  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* gdb.python/py-inferior.exp: Add test cases for 'connection_num'.