Panoptescli

- Update: Replace pkg_resources with importlib.metadata
- New: Add new package versions to info for debugging
- New: Add subject update-metadata command
- New: Add batch Aggregation

- Update panoptes-client requirement to >=1.6
- Bump humanize to <4.8
- Fix: Remove deprecated U mode in file open
- Update README with iNat and python 2 deprecation info.

- New: support `%` in manifest column headings for indexed subject sets. Manifest headers with the `%` prefix will automatically be added to the subject set configuration `'indexFields'` list.
- New: iNaturalist observation import functionality (see --help for details)
- Update reqs for humanize, click, pyyaml, & pathvalidate

- Update panoptes-client requirement to >=1.3

- Bump pyyaml to <5.4
- Bump humanize to <1.1

- Fix: Bump pyyaml requirement to >=5.1 to fix AttributeError