net-mgmt/py-napalm: Update to 2.4.0

While I'm here:

• Add location to license file

Changelog:

• Many bugfixes and improvements (total count of 42 to be exact)

https://github.com/napalm-automation/napalm/releases/tag/2.4.0

Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D19439