Pysnmp 6.2.2, 5.1.0 broke compatibility

[number492]

Pysnmp has been taken over [1], and the new repo's owners [2] have been releasing new versions under a different package name. The package name have been changed recently [3], and now robotframework-snmplibrary is not compatible with the latest pysnmp.

1: etingof/pysnmp#429
2: https://github.com/lextudio/pysnmp
3: https://docs.lextudio.com/pysnmp/upgrade