Python 3.10 + 3.11 support

I know that Python 3.10 stable isn't out yet, but it'd be nice to be ready for it if possible.

https://docs.python.org/3.10/whatsnew/3.10.html

Currently, setup.py sets the version range at 3.6-3.9.

https://github.com/google/fhir/blob/master/py/setup.py#L155

```
$ poetry update
Updating dependencies
Resolving dependencies... (8.4s)

SolverProblemError

The current project's Python requirement (>=3.8,<3.11) is not compatible with some of the required packages Python requirement:
- google-fhir requires Python >=3.6, <3.10, so it will not be satisfied for Python >=3.10,<3.11

Because google-fhir (0.7.2) requires Python >=3.6, <3.10
and no versions of google-fhir match >0.7.2,<0.8.0, google-fhir is forbidden.
So, because hos depends on google-fhir (^0.7.2), version solving failed.
```