test_runner: Support only Django version >= 1.11.

In previous versions, the argument to run_subsuite is a tuple of
length 3, while in versions >= 1.11 it's a tuple of length 4.

zerver/lib/test_runner.py

@@ -240,31 +240,15 @@ def process_instrumented_calls(func):
         func(call)
 
 SerializedSubsuite = Tuple[Type[Iterable[TestCase]], List[str]]
-SubsuiteArgs = Union[
-    Tuple[Type['RemoteTestRunner'], int, SerializedSubsuite, bool],
-    Tuple[int, SerializedSubsuite, bool]
-]
+SubsuiteArgs = Tuple[Type['RemoteTestRunner'], int, SerializedSubsuite, bool]
 
 def run_subsuite(args):
     # type: (SubsuiteArgs) -> Tuple[int, Any]
     # Reset the accumulated INSTRUMENTED_CALLS before running this subsuite.
     test_helpers.INSTRUMENTED_CALLS = []
-    if len(args) == 4:
-        # This condition runs for Django 1.11. The first argument is the
-        # test runner class but we don't need it because we run our own
-        # version of the runner class.
-        args = cast(Tuple[Type[RemoteTestRunner],
-                          int,
-                          SerializedSubsuite,
-                          bool], args)
-        _, subsuite_index, subsuite, failfast = args
-    else:
-        # We are making mypy to ignore the following line because
-        # it is only valid in Django 1.10. In Django 1.11, this
-        # tuple is made up of 4 elements.
-        args = cast(Tuple[int, SerializedSubsuite, bool], args)
-        subsuite_index, subsuite, failfast = args
-
+    # The first argument is the test runner class but we don't need it
+    # because we run our own version of the runner class.
+    _, subsuite_index, subsuite, failfast = args
     runner = RemoteTestRunner(failfast=failfast)
     result = runner.run(deserialize_suite(subsuite))
     # Now we send instrumentation related events. This data will be