Upgrade available Python SDL version to 3.0.0

Update XAPP-FRAME-PY module dependencies to use latest SDL version 3.0.0
(RIC-699) what contains support for SDL multi-namespace and sentinel based
DB capacity scaling features. Also Notification packing has been fixed in
SDL version 3.0.0 (RIC-795).
XAPP-FRAME API change: received SDL events are now returned as a list of
strings, earlier only one event of type string was possible to return.
This change applies to following XAPP-FRAME APIs: 'handle_events()' and
a callback function registered by the 'subscribe_channel()' call.
Bump XAPP-FRAME-PY version to 2.0.0.

Issue-ID: RIC-699

Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: Ica3c89d5a47451874fbc5ccd9a6b021fd30f6a53