Fix ordering of mixed stdout / stderr printing

testing/resources/stdout_stderr_repo/.pre-commit-hooks.yaml

@@ -0,0 +1,4 @@
+-   id: stdout-stderr
+    name: stdout-stderr
+    language: script
+    entry: ./entry

testing/resources/stdout_stderr_repo/entry

@@ -0,0 +1,13 @@
+#!/usr/bin/env python
+import sys
+
+
+def main():
+    for i in range(6):
+        f = sys.stdout if i % 2 == 0 else sys.stderr
+        f.write('{}\n'.format(i))
+        f.flush()
+
+
+if __name__ == '__main__':
+    exit(main())