Fix and enable unused-variable lint

In _server.py start_server_batch_result is removed because
start_server_batch can only ever fail as a result of a programming
defect in gRPC Python and not the application. This differs from some
analogous-appearing points in _channel.py where the result of
start_client_batch is checked because at those points it is possible
for a failure to indicate a programming defect on the part of the
application.

.pylintrc

+[VARIABLES]
+# TODO(https://github.com/PyCQA/pylint/issues/1345): How does the inspection
+# not include "unused_" and "ignored_" by default?
+dummy-variables-rgx=^ignored_|^unused_
+
 [MESSAGES CONTROL]
 
 #TODO: Enable missing-docstring
@@ -15,7 +20,6 @@
 #TODO: Enable wrong-import-order
 #TODO: Enable no-value-for-parameter
 #TODO: Enable cyclic-import
-#TODO: Enable unused-variable
 #TODO: Enable redefined-outer-name
 #TODO: Enable too-many-instance-attributes
 #TODO: Enable broad-except
@@ -29,4 +33,4 @@
 #TODO: Enable too-many-nested-blocks
 #TODO: Enable super-init-not-called
 
-disable=missing-docstring,too-few-public-methods,too-many-arguments,no-init,duplicate-code,invalid-name,suppressed-message,locally-disabled,protected-access,no-name-in-module,unused-argument,fixme,wrong-import-order,no-value-for-parameter,cyclic-import,unused-variable,redefined-outer-name,too-many-instance-attributes,broad-except,too-many-locals,too-many-lines,redefined-variable-type,next-method-called,import-error,useless-else-on-loop,too-many-return-statements,too-many-nested-blocks,super-init-not-called
+disable=missing-docstring,too-few-public-methods,too-many-arguments,no-init,duplicate-code,invalid-name,suppressed-message,locally-disabled,protected-access,no-name-in-module,unused-argument,fixme,wrong-import-order,no-value-for-parameter,cyclic-import,redefined-outer-name,too-many-instance-attributes,broad-except,too-many-locals,too-many-lines,redefined-variable-type,next-method-called,import-error,useless-else-on-loop,too-many-return-statements,too-many-nested-blocks,super-init-not-called

src/python/grpcio/grpc/_channel.py

@@ -200,7 +200,7 @@ def _consume_request_iterator(request_iterator, state, call,
                 request = next(request_iterator)
             except StopIteration:
                 break
-            except Exception as e:
+            except Exception:
                 logging.exception("Exception iterating requests!")
                 call.cancel()
                 _abort(state, grpc.StatusCode.UNKNOWN,

src/python/grpcio/grpc/_server.py

@@ -342,7 +342,7 @@ def _unary_request(rpc_event, state, request_deserializer):
             if state.client is _CANCELLED or state.statused:
                 return None
             else:
-                start_server_batch_result = rpc_event.operation_call.start_server_batch(
+                rpc_event.operation_call.start_server_batch(
                     cygrpc.Operations(
                         (cygrpc.operation_receive_message(_EMPTY_FLAGS),)),
                     _receive_message(state, rpc_event.operation_call,

src/python/grpcio/grpc/framework/foundation/logging_pool.py

@@ -39,7 +39,7 @@ def _wrap(behavior):
     def _wrapping(*args, **kwargs):
         try:
             return behavior(*args, **kwargs)
-        except Exception as e:
+        except Exception:
             logging.exception(
                 'Unexpected exception from %s executed in logging pool!',
                 behavior)