minimize install dependencies

[kapilt]

ideally just lark and re2 for core celpython is needed, Dev dependencies (including sphinx, pytest, etc) should be moved over to a dev dependency group. the custodian integration probably could be archived at this point on a tag (pyyaml, c7n, etc), or made an optional dependency.

given re2 is required for correctness, its not clear that it should be considered an optional dependency.

[slott56]

Closed with #119