devel/py-jupyter_client: Update to 6.1.0

• Add test target.
• Remove support for Python 2.x.
• Update consumers to use Python >= 3.5 (avoid break bulk -a).

Changelog: https://github.com/jupyter/jupyter_client/blob/6.1.0/docs/changelog.rst

PR: 244933
Approved by: koobs (python, maintainer)