Upgrade pysnmp to 4.4.3

[fabaff]

Description

Changelog: https://github.com/etingof/pysnmp/blob/master/CHANGES.txt

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"

Checklist:

If the code communicates with devices, web services, or third-party tools:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• New dependencies have been added to the REQUIREMENTS variable ([example][ex-requir]).
• New dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.