Upgrade lxml to 4.6.5

[obynio]

Thanks for your reply @armab, here is the PR you asked me about.

This PR fixes #71 by upgrading the lxml dependency.
Installation of the pack on Ubuntu 20 was successful and I was able to run actions and send messages to Slack.

Changelog

• Upgrade lxml dependency from 3.8.0 to 4.6.5

[arm4b]

From the CI failure https://github.com/StackStorm-Exchange/stackstorm-slack/runs/4729962031:

sensors/slack_sensor.py:4:0: E0401: Unable to import 'slackclient' (import-error)

Something changed between the major slack versions.

[arm4b]

Looks like the sensor here https://github.com/StackStorm-Exchange/stackstorm-slack/blob/master/sensors/slack_sensor.py is the only file that really uses slackclient.

Everything else is just a direct HTTP API call via requests https://github.com/StackStorm-Exchange/stackstorm-slack/blob/master/actions/run.py and so doesn't depend on slackclient from pip.

If you don't want to rewrite the entire slack sensor with the new API, I think just bumping the slackapi to latest 1.x version should pass the CI and be something working to merge the PR today.

If anyone will be willing to update the sensor, - probably it's best to do in the future PR.

[obynio]

Thanks for your review @armab for the moment let's just upgrade the lxml dependency so that the package can work on Ubuntu 20 👍

[arm4b]

Thanks for the contribution! 👍

[arm4b]

I also updated the version here.
Otherwise, new pack won't be released.