Yum update problems - public keys missing

How do I get past update problems where there is a public key missing. Here is an example:

warning: /var/cache/yum/x86_64/7/prod-centos7-scl/packages/python27-python-2.7.18-4.el7.x86_64.rpm: Header V4 RSA/SHA256 Signatu, key ID f2exxxx: NOKEY

Public key for python27-python-devel-2.7.18-4.el7.x86_64.rpm is not installed

I see this frequently but do not know how to get or generate the keys.