mypy: Use python3 type syntax in test-backend.

tools/test-backend

@@ -145,8 +145,7 @@ enforce_fully_covered = sorted(target_fully_covered - not_yet_fully_covered)
 
 FAILED_TEST_PATH = 'var/last_test_failure.json'
 
-def get_failed_tests():
-    # type: () -> List[str]
+def get_failed_tests() -> List[str]:
     try:
         with open(FAILED_TEST_PATH, 'r') as f:
             return ujson.load(f)
@@ -154,14 +153,12 @@ def get_failed_tests():
         print("var/last_test_failure.json doesn't exist; running all tests.")
         return []
 
-def write_failed_tests(failed_tests):
-    # type: (List[str]) -> None
+def write_failed_tests(failed_tests: List[str]) -> None:
     if failed_tests:
         with open(FAILED_TEST_PATH, 'w') as f:
             ujson.dump(failed_tests, f)
 
-def block_internet():
-    # type: () -> None
+def block_internet() -> None:
     # We are blocking internet currently by assuming mostly any test would use
     # httplib2 to access internet.
     requests_orig = requests.request
@@ -300,8 +297,7 @@ def main() -> None:
         for i, suite in enumerate(args):
             args[i] = suite.rstrip('/').replace("/", ".")
 
-        def rewrite_arguments(search_key):
-            # type: (str) -> None
+        def rewrite_arguments(search_key: str) -> None:
             for root, dirs, files_names in os.walk(zerver_test_dir, topdown=False):
                 for file_name in files_names:
                     # Check for files starting with alphanumeric characters and ending with '.py'