build: Support Python 3.9+

[matthewfeickert]

Resolves #158

• Require Python 3.9+, dropping support for Python 3.7 and 3.8 as both are EOL, and Python 3.8+ syntax is already being used in the current release which already EOLs 3.7.
• Update minimum Python used in CI to 3.9.

There are stil Python 3.7 specific notes in the code:

$ git grep "3\.7"
func_adl/ast/function_simplifier.py:442:        # Get the value out - this is due to supporting python 3.7-3.9
func_adl/ast/function_simplifier.py:567:    """Deal with 3.7, and 3.8 differences in how indexing for list and tuple
func_adl/type_based_replacement.py:946:            "3.7 compatibility"
func_adl/type_based_replacement.py:951:            "3.7 compatibility"
func_adl/type_based_replacement.py:956:            "3.7 compatibility"
func_adl/util_types.py:25:    * This works for 3.7 forward (but not back!)

but I don't know enough about func-adl internals to be able to edit these quickly, so I'll need @gordonwatts or @masonproffitt to look at that.