ci - move linting to 3.7 and remove flake8 pin (#3996)
ci - move linting to 3.7 and remove flake8 pin (#3996)


FireballDWF/cloud-custodian,thisisshi/cloud-custodian,ewbankkit/cloud-custodian,capitalone/cloud-custodian,FireballDWF/cloud-custodian,Sutto/cloud-custodian,alfredgamulo/cloud-custodian,Sutto/cloud-custodian,thisisshi/cloud-custodian,ocampocj/cloud-custodian,ewbankkit/cloud-custodian,kapilt/cloud-custodian,capitalone/cloud-custodian,ocampocj/cloud-custodian,Sutto/cloud-custodian,capitalone/cloud-custodian,ocampocj/cloud-custodian,capitalone/cloud-custodian,Sutto/cloud-custodian,alfredgamulo/cloud-custodian,FireballDWF/cloud-custodian,thisisshi/cloud-custodian,ocampocj/cloud-custodian,kapilt/cloud-custodian,ewbankkit/cloud-custodian,ewbankkit/cloud-custodian,kapilt/cloud-custodian,alfredgamulo/cloud-custodian,FireballDWF/cloud-custodian,kapilt/cloud-custodian