coleifer/peewee 3.3.0 on GitHub

Added support for SQLite's new ON CONFLICT clause, which is modelled on the syntax used by Postgresql and will be available in SQLite 3.24.0 and onward.
Added better support for using common table expressions and a cleaner way of implementing recursive CTEs, both of which are also tested with integration tests (as opposed to just checking the generated SQL).
Modernized the CI environment to utilize the latest MariaDB features, so we can test window functions and CTEs with MySQL (when available).
Reorganized and unified the feature-flags in the test suite.