v0.19.1 - Another Quality-of-Live release

This version provides three key improvements:
(we skipped v0.19.0 because we are stupid)

• Removed the strategies package in favor of lazy imports directly from the optimizer package - just do pb.opt.ues
• Simplified the access to frequently-used parts of the query predicates. This should combat the excessive need to type query.predicates() everywhere. Instead, we can now use query.joins_between(), etc. directly.
• Added a new join_equivalence_classes() introspection method to the database schema. This should especially aid optimization approaches that rely on an a-priori knowledge of the possible joins in a schema. While it is not an 100% replacement of leaking the evaluation workload, it does come pretty close.
• postgres.connect() now supports obtaining the connection information from environment variables (e.g. PGDATABASE)

Lastly, PostBOUND is now available directly from PyPI, no need for the manual installation anymore!