Replace pypi:pyrfc3339

### 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

```python
clean internal types
```
