Uninitialized attributes in BaseClient

[tabinfl]

Describe the bug
Calling is_connected() before a connection attempt accesses uninitialized attribute.

To Reproduce
Call is_connected() before calling connect():

[...]/sliver/client.py", line 52, in is_connected
    return self._channel is not None
AttributeError: 'SliverClient' object has no attribute '_channel'

Fix
Quick fix to BaseClient's __init__ (initialize SliverRPCStub also for good measure):

        self._channel: grpc.aio.Channel = None
        self._stub: SliverRPCStub = None