Use pip installed so3g in GH Action workflows

[BrianJKoopman]

Description

Now that so3g is easily pip installable this allows us to do several things:

• Install so3g within the GH Actions workflows directly, meaning we can drop the use of docker containers for running tests that depend on spt3g/so3g.
• Remove the spt3g marker on our tests used to select only those test which do or do not depend on spt3g/so3g. We now just insist that so3g is installed for testing by adding it to the requirements/testing.txt file. Associated with this we no longer check for successful imports as a dependency for running those tests, meaning we can also drop the pytest-dependency module as a requirement.

I've also listed so3g as an optional requirement via extras_install in setup.py.

Motivation and Context

Take advantage of the fact that so3g is now on PyPI and easily installable.

How Has This Been Tested?

I've tested the test locally, and they will run in this PR/have run on these commits. The deploy develop and deploy workflow changes are currently untested, though mimic those changes made in the pytest workflow.

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.

[mhasself]

Neat!