Prep for conda-forge migration

[pluflou]

This PR:

• updates the package sdist to include the tests folder so we can run tests at conda build time
• removes the conda deployment to the slacgismo channel
• removes support for Python <=3.9 (see status here)
• adds __version__ attribute

Checklist for PR authors (skip items if you don't have permissions or they are not applicable)

• N/A Updated or added relevant tests
• N/A Updated relevant documentation: docs will be updated once deployed on conda-forge
• N/A Added relevant label(s)
• N/A All comments are resolved

[pluflou]

@bmeyers I suggest we merge this now and make a patch release, then finalize adding the package to conda-forge. Once on conda-forge, we should see if it's possible to update the package version being reviewed in pyOpenSci/software-submission#210