Retire py 3.6. Add py 3.10 trove classifiers

No functional change.

Just updates trove classifiers for pypi.

slack api 2: no functional change

• no functional change
• update slack api to 2.x
• update pypyr docs to point to new https://pypyr.io website
• pypi author change
• move from shippable to GitHub Actions.

update slack api to 2.x - no functional change

No functional change in this one - just updating the underlying slack api to use slack's 2.x specification.

no functional change - pypi deployment updates

• pypi url upgrade to new pypi repo
• no functional change - upgrade beta to v1.0 release

inherit all exceptions from pypyr PlugInError

• inherit all exceptions from pypyr PlugInError
• minor typo correction in README

amend keywords in pypi

• Change keywords for better discover-ability as a pypyr plug-in on pypi
• version bump

BREAKING CHANGE: Rename pypi from pypyr-slack to pypyrslack

• Breaking change. Henceforth, use pip install pypyrslack, not pip install pypyr-slack

KeyInContextHasNoValueError, KeyNotInContextError

• use new pypyr error types for context validation errors
• unit tests
• version bump

Remove references to --name

• pypyr cli now invokes as pypyr pipe rather than pypyr --name pipe
• fix some README grammar
• bump dev status to beta, like pypyr core

add auto pypi release

no end-user impact