fix: preserve ClientSession constructor compatibility

Author: asher-pem-arm

labgrid/remote/client.py

@@ -93,7 +93,11 @@ class ClientSession:
     the coordinator."""
 
     address = attr.ib(validator=attr.validators.instance_of(str))
-    credentials = attr.ib(validator=attr.validators.optional(attr.validators.instance_of(grpc.ChannelCredentials)))
+    credentials = attr.ib(
+        default=None,
+        kw_only=True,
+        validator=attr.validators.optional(attr.validators.instance_of(grpc.ChannelCredentials)),
+    )
     loop = attr.ib(validator=attr.validators.instance_of(asyncio.BaseEventLoop))
     env = attr.ib(default=None, validator=attr.validators.optional(attr.validators.instance_of(Environment)))
     role = attr.ib(default=None, validator=attr.validators.optional(attr.validators.instance_of(str)))
@@ -1721,6 +1725,8 @@ def start_session(
     Args:
         address: coordinator address as HOST[:PORT], PORT defaults to 20408
         extra: additional kwargs for ClientSession
+        credentials: optional gRPC channel credentials; when None, use an
+              insecure channel
         debug: set debug mode of the event loop
         loop: explicit event loop to use (otherwise a previously stashed loop,
               if retrievable the current thread's loop or a new loop is used)
@@ -1735,7 +1741,7 @@ def start_session(
 
     address = proxymanager.get_grpc_address(address, default_port=20408)
 
-    session = ClientSession(address, credentials, loop, **extra)
+    session = ClientSession(address, loop, credentials=credentials, **extra)
     loop.run_until_complete(session.start())
     return session