chore: upgrade python3.9 for running appinspect cli

[dfederschmidt]

AppInspect v3.0.0 shows the following change:

Mainly, this means changing os.path.join() to any of its object-oriented alternative, e.g. pathlib.Path(). Refer to the official pathlib migration guide.

Now, the error I'm seeing is

      Check that all XML files are well-formed. 
          ERROR: 'PosixPath' object has no attribute 
              'getCharacterStream' 

Which leads me to believe that we're encountering this issue because the action runs the command with Python 3.7 and the fix for this issue only landed in 3.8.
Could it be that this makes appinspect-cli==3.0.0 partially incompatible with 3.7?

From what I've been seeing, the appinspect-cli command is independent of any Python version that Splunk uses, so we can upgrade the runtime to Python 3.9.

@artemrys I'm not sure how to upgrade the image path in lockstep with the release so marking this as a draft for now.
Also, I have not tested this change yet.

appinspect-cli-action/action.yml

Line 38 in bf9e3bc

image: "docker://ghcr.io/splunk/appinspect-cli-action/appinspect-cli-action:v2.0.0"