Switch from arrow to pendulum

[sscherfke]

trafficstars

This should fix #6. Also see http://blog.eustace.io/please-stop-using-arrow.html, https://pendulum.eustace.io/blog/a-faster-alternative-to-pyz.html and https://twitter.com/i/web/status/944203346738638848 ;-)

[cleebp]

Was running into issues with jinja2-time not being compatible with more recent arrow builds (v17), found this PR and applied the changes locally in a patch build from source and now I'm able to use jinja2-time + pendulum for my jupyterlab extension templating. Just wanted to say thanks for putting this up!