(maint) update pyenv to 3.7.13
(maint) update pyenv to 3.7.13

Version 3.7.6 is no longer available on travis, so this updates to
3.7.13

puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb