Add missing return type for firebase_admin.firestore.client() (#747)

firestore.client() should have return type google.cloud.firestore.Client

Co-authored-by: Lahiru Maramba <[email protected]>

Author: wliumelb

firebase_admin/firestore.py

@@ -34,7 +34,7 @@
 _FIRESTORE_ATTRIBUTE = '_firestore'
 
 
-def client(app=None):
+def client(app=None) -> firestore.Client:
     """Returns a client that can be used to interact with Google Cloud Firestore.
 
     Args: