Rename to py-snmpclitools (per upstream) and upgrade to 0.6.1.
Enable concurrent installation (USE_PYTHON=concurrent).

PR: 235772
Submitted by: John W. O'Brien
Reviewed by: ler (mentor)
Approved by: mhjacks@swbell.net (maintainer), ler (mentor)
Differential Revision: https://reviews.freebsd.org/D19526