pymarkdown requires 3.8, address or document

[scop]

Describe the bug

As noted in https://github.com/scop/bash-completion/pull/628#issuecomment-941905119, pymarkdown requires Python 3.8, but our intent has been for the dev/test setup to not require newer Python than 3.6.

See if there is something we could do about this, falling back to documenting if not.

We don't have to make pre-commit tests work on all tested distros, but the pytest tests should continue working with >= 3.6 because getting a newer one installed on some older distros we test with is a bit difficult/clunky.

To reproduce

1. cd to '...'
2. Type '....'
3. See problem

Expected behavior

Versions (please complete the following information)

• [ ] Operating system name/distribution and version:
• [ ] bash version, echo "$BASH_VERSION":
• [ ] bash-completion version, (IFS=.; echo "${BASH_COMPLETION_VERSINFO[*]}"):

Additional context

Debug trace