net-mgmt/py-junos-eznc: Update to 2.7.6

• Switch to the PEP517 build framework to make it future-proof.

Changelog since 2.7.0:

https://github.com/Juniper/py-junos-eznc/blob/2.7.6/RELEASE-NOTES.md

PR: 293458
Approved by: maintainer timeout