www/py-urlwatch: Update to 2.25

• Fix BUILD if DOCS option is unset as BINARY_ALIAS can only be used if textproc/py-sphinx (enabled via the DOCS option) is present.

• Add EXAMPLES option to let users choose to install the supplied examples.

Changelog:

https://github.com/thp/urlwatch/blob/2.25/CHANGELOG.md

MFH: 2022Q2
(cherry picked from commit b693974d417d2751131991f2fb78f790da998817)