Remove zerver/lib/parallel.py from mypy's exclude_py3.

zerver/lib/parallel.py

@@ -28,7 +28,7 @@ def run_parallel(job, data, threads=6):
             except OSError as e:
                 if e.errno != errno.EBADF:
                     raise
-            sys.stdin = open("/dev/null", "r")
+            sys.stdin = open("/dev/null", "r") # type: ignore # py2 and py3 stubs are not compatible
             os._exit(job(item))
 
         pids[pid] = item