BUG: Fix parsing of ISO8601 durations
This PR fixes the following behavior:

- Adds "W" as a valid weeks designator
- Fixes parsing of durations with empty periods (e.g. "PT10M")
- Fixes partial parsing of durations (e.g. "P1DT1H")

Fixes #29773.
Fixes #36204.

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry