Update dependency google-cloud-core to v1.6.0

Author: renovate-bot

requirements.txt

@@ -4,7 +4,7 @@ cffi==1.14.2
 chardet==3.0.4
 google-api-core==1.22.1
 google-auth==1.20.1
-google-cloud-core==1.4.1
+google-cloud-core==1.6.0
 google-cloud-storage==1.30.0
 google-crc32c==0.1.0
 google-resumable-media==0.7.1