Make AsyncTransport and Transport proper abstract base classes (#350)

Author: willfrey

gql/transport/async_transport.py

@@ -4,7 +4,7 @@
 from graphql import DocumentNode, ExecutionResult
 
 
-class AsyncTransport:
+class AsyncTransport(abc.ABC):
     @abc.abstractmethod
     async def connect(self):
         """Coroutine used to create a connection to the specified address"""

gql/transport/transport.py

@@ -3,7 +3,7 @@
 from graphql import DocumentNode, ExecutionResult
 
 
-class Transport:
+class Transport(abc.ABC):
     @abc.abstractmethod
     def execute(self, document: DocumentNode, *args, **kwargs) -> ExecutionResult:
         """Execute GraphQL query.