enqueue_file.py: Add type hint for mypy.

tools/run-mypy

@@ -41,7 +41,6 @@ exclude_py2 = []
 
 exclude_py3 = """
 zerver/lib/ccache.py
-zerver/management/commands/enqueue_file.py
 zerver/management/commands/rename_stream.py
 zerver/management/commands/runtornado.py
 zerver/tests/test_i18n.py

zerver/management/commands/enqueue_file.py

@@ -1,7 +1,7 @@
 from __future__ import absolute_import
 from __future__ import print_function
 
-from typing import Any
+from typing import Any, IO
 
 from argparse import ArgumentParser
 from django.core.management.base import BaseCommand
@@ -39,7 +39,7 @@ You can use "-" to represent stdin.
         file_name = options['file_name']
 
         if file_name == '-':
-            f = sys.stdin
+            f = sys.stdin # type: IO[str]
         else:
             f = open(file_name)