fix: reseed for each auto id on 3.6 to avoid collisions (#388)

* fix: Fixes #346 by reseeding for each auto id on py3.6

google/cloud/firestore_v1/base_collection.py

@@ -14,6 +14,7 @@
 
 """Classes for representing collections for the Google Cloud Firestore API."""
 import random
+import sys
 
 from google.api_core import retry as retries  # type: ignore
 
@@ -455,6 +456,12 @@ def _auto_id() -> str:
         str: A 20 character string composed of digits, uppercase and
         lowercase and letters.
     """
+    if sys.version_info < (3, 7):
+        # TODO: remove when 3.6 support is discontinued.
+        # On python 3.6, random will provide the same results when forked. Reseed
+        # on each iteration to avoid collisions.
+        random.seed()
+
     return "".join(random.choice(_AUTO_ID_CHARS) for _ in range(20))