Cutting release google-cloud-core==0.26.0. (#3727)
Cutting release google-cloud-core==0.26.0. (#3727)


dhermes/google-cloud-python,dhermes/gcloud-python,jonparrott/google-cloud-python,tseaver/gcloud-python,googleapis/google-cloud-python,tseaver/google-cloud-python,dhermes/gcloud-python,GoogleCloudPlatform/gcloud-python,tartavull/google-cloud-python,tseaver/google-cloud-python,calpeyser/google-cloud-python,tswast/google-cloud-python,dhermes/google-cloud-python,jonparrott/google-cloud-python,tseaver/google-cloud-python,calpeyser/google-cloud-python,tseaver/gcloud-python,jonparrott/gcloud-python,googleapis/google-cloud-python,GoogleCloudPlatform/gcloud-python,tartavull/google-cloud-python,dhermes/google-cloud-python,jonparrott/gcloud-python,tswast/google-cloud-python,tswast/google-cloud-python