Add missing init hook

http://python-notes.curiousefficiency.org/en/latest/python_concepts/import_traps.html#the-init-py-trap

pre_commit_hooks/check_missing_inits.py

@@ -0,0 +1,25 @@
+import os
+from argparse import ArgumentParser
+from typing import Optional
+from typing import Sequence
+
+
+def main(argv=None):  # type: (Optional[Sequence[str]]) -> int
+    parser = ArgumentParser()
+    parser.add_argument('filenames', nargs='*', help='Filenames to check')
+    args = parser.parse_args(argv)
+
+    directories = {os.path.dirname(f) for f in args.filenames}
+    missing_dirs = set()
+    for d in directories:
+        if not os.path.exists(os.path.join(d, '__init__.py')):
+            missing_dirs.add(d)
+
+    for d in sorted(missing_dirs):
+        print('No __init__.py file found in: {}'.format(d))
+
+    return 1 if len(missing_dirs) else 0
+
+
+if __name__ == '__main__':
+    exit(main())