Remove casts which don't work correctly because MyPy doesn't inherit

parent class signature for methods which are overriden in the subclass.

This means we sadly need to rely on annotations from the base class.

See python/mypy#3903 for details.

example_compute.py

@@ -21,14 +21,8 @@
 
 from typing import Type, cast
 
-ec2_cls = get_driver(Provider.EC2)
-rackspace_cls = get_driver(Provider.RACKSPACE)
-
-# NOTE: If you are using driver methods which are not part of the standard API,
-# you need to explicitly cast the driver class reference to the correct class
-# for type checking to work correctly
-EC2 = cast(Type[EC2NodeDriver], ec2_cls)
-Rackspace = cast(Type[RackspaceNodeDriver], rackspace_cls)
+EC2 = get_driver(Provider.EC2)
+Rackspace = get_driver(Provider.RACKSPACE)
 
 drivers = [EC2('access key id', 'secret key', region='us-east-1'),
            Rackspace('username', 'api key', region='iad')]