chore: pin new release (#3688)

Co-authored-by: larkee <[email protected]>

Author: larkee

spanner/cloud-client/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-spanner==1.15.1
+google-cloud-spanner==1.16.0
 futures==3.3.0; python_version < "3"