chalice support python 3.9 as host python

issue and solution documented in https://github.com/aws/chalice/issues/1696
specifically re issue https://github.com/aws/chalice/issues/1696#issuecomment-817369468
specifically re solution, pass --python-version 38 to pip (or alternative lambda runtime version).

tldr, chalice doesn't properly package cross python version dependencies.

Start with linked issue 1696 and its referenced comment, then trace Chalice's dependency-packaging path for handling the host Python version and the Lambda runtime version. Verify the package produced for a Python 3.9 host includes dependencies compatible with the selected runtime, including the documented --python-version 38 case.

Written by the indexing model from the issue text.