Update dependency google-api-core to v1.26.3

Author: renovate-bot

requirements.txt

@@ -2,7 +2,7 @@ cachetools==4.1.1
 certifi==2020.6.20
 cffi==1.14.2
 chardet==3.0.4
-google-api-core==1.22.1
+google-api-core==1.26.3
 google-auth==1.20.1
 google-cloud-core==1.4.1
 google-cloud-storage==1.30.0