cel-python without binary dependencies

[dbohdan]

I am interested in cel-python as a potential replacement for simpleeval. I want to use it in a small utility distributed as a platform-agnostic zipapp built with shiv. cel-python seems to have one direct, non-development binary dependency, PyYAML. PyYAML increases the size of the zipapp and, more importantly, prevents you from making it platform-agnostic. In a quick test, my utility was around 11 MiB after adding cel-python from PyPI and around 0.5 MiB after adding a modified version without PyYAML. Could you make PyYAML an optional dependency for use cases like mine?

[kapilt]

addressed in #87