Update CI scaffolding

[jbusecke]

• Update Linter config Update Linters #359
• Update release workflow Modernized release workflow #361
• Test newer python versions Update Mamba based CI #362
• Consolidate all the packging info into a pyproject.toml
• Fix Docs build Docs build broken  #276
• Unpin xgcm
• Trim CI to a pip only build to make CI faster (xesmf is too annoying to build with pip?)

[rsignell]

@jbusecke could we get a new release so that these fixes are included on conda-forge? (especially the removal of the xgcm pin!)

[jbusecke]

Of course, once I have implemented Ill release this asap!