net-mgmt/py-napalm: Update to 3.1.0

Changelog:

Core changes:

• Fix: force_no_enable enabled on ios and nx only
• lxml text retrieval (find_txt) shouldn't fail if no text element available

NX-OS:

• Initial version for nxos_ssh get_optics
• Fixed get_bgp_neighbors and asdot notation on NX-OS

Junos:

• Fix "KeyError in get_environment() on Juniper MX10k3 with Junos 19.2R1.8 by making PEM naming consistent

IOS

• Fix get_optics metric
• Allow username detection on older Cisco ios versions

https://github.com/napalm-automation/napalm/releases/tag/3.1.0

MFH: 2020Q3