Replace pypi:pyrfc3339

[dimaqq]

Description

pyrfc3339 has not seen an update since 2018, and is not typed.

we should replace it.

I think https://pypi.org/project/ciso8601/ is a reasonable candidate, better maintained (updated last year), supports RFC 3339 format (in addition to ISO 8601).

Urgency

It would save from a weird workaround

Code I'd Like to Run

clean internal types

[dimaqq]

removing assignment, given that the issue is low priority.

@james-garner-canonical this could be a nice little self-contained low-hanging fruit.

[dimaqq]

Done in #1247