Upgrade pysnmp to 4.3.7

[fabaff]

4.3.7

• This is a patch release addressing bad import accidentally slipped into pre-release code.

Tested with the following configuration:

sensor:
  - platform: snmp
    name: SNMP TEST String
    host: demo.snmplabs.com
    community: public
    baseoid: 1.3.6.1.4.1.2021.10.1.3.1
    unit_of_measurement: "load"

[mention-bot]

@fabaff, thanks for your PR! By analyzing the history of the files in this pull request, we identified @kellerza, @persandstrom and @balloob to be potential reviewers.