Prepare for v0.9.0 release

[BrianJKoopman]

Description

Some final things before issuing the next release. Add a PyPI badge (should have done this in #243) and bump the so3g version to get the latest so3g updates.

Motivation and Context

Preparing for a release.

How Has This Been Tested?

CI tests only.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• Unless I am preparing a release, I have opened this PR onto the develop branch.