doc: announce python 2 support removal

doc: announce python 2 support removal

Now that Python 2 is officially not maintained anymore, prepare the
removal of support in DPDK.

Add a deprecation notice indicating the removal schedule.

Signed-off-by: Robin Jarry <robin.jarry@6wind.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Kevin Traynor <ktraynor@redhat.com>
Acked-by: David Marchand <david.marchand@redhat.com>