Upgrade to cel-rust 0.14, extended stdlib, and PyO3 0.29 (#37)

Upgrade cel-rust 0.13->0.14 and PyO3 0.27->0.29 (fixes two upstream advisories), add an opt-in extended standard library mirroring cel-go, expose expression static analysis (Program.references), and prepare 0.7.0. See CHANGELOG for behaviour changes and details.

Prepare 0.6.0 release

Fix mapping conversion for dict subclasses (#29)

* Fix mapping conversion for dict subclasses

* Bump version to 0.5.6

* Fix dict subclass test to avoid shared state

* Clarify mapping key conversion error

Prepare 0.5.5 release

Add substring() function to stdlib and improve logging (#21)

* Remove warning log
* Add substring function to stdlib, use stdlib in CLI

Release version 0.5.1

Release/0.5.0 (#8)

* documentation updates
* Add version retrieval and display option to CLI
* bump version to 0.5.0

Fix regex pattern in CLI

Update tests/test_arithmetic.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

handle None values