www/py-urlwatch: update to v2.23, add DOCS

Add DOCS option (uses py-sphinx and GNU make) to build/install
html and text documentation.

Ensure build works for non-default Python flavors.

Update TEST_DEPENDS to list additional self-test requisites.

Changelog: https://github.com/thp/urlwatch/blob/2.23/CHANGELOG.md#223----2021-04-10

PR: 256085

Approved by: kai@FreeBSD.org