MAINT: Migrate to github actions

[jbellister-slac]

Context

This PR removes the azure pipelines files, and replaces them with workflows for github actions. The intent is to keep the majority of the current functionality intact, but get to a state where maintenance and future updates are now much easier. There are some changes:

• The test suite now runs against multiple versions of pyqt as was suggested in BLD: pin pyqt to 5.x #999
  5.12.3 is the one with designer support, 5.15.7 is the latest from conda-forge
• Switched from conda to mamba to reduce solve time for testing and documentation building
• Moved to trusted publishing for uploading to PyPI. This recently came out of beta and is now recommended as best practice (https://docs.pypi.org/trusted-publishers/)

The one workflow I haven't migrated over yet is the one that uploads to the pydm tag in anaconda. I'm not sure if we still want to maintain these separate tags with pydm available on conda-forge

https://anaconda.org/pydm-tag/pydm/files
https://anaconda.org/pydm-dev/pydm/files

Testing

All new files were tested on my fork.

This PR will kick off the new workflow for running tests.

The documentation job was tested here, though just uploading the built docs locally rather than pushing to gh-pages:
https://github.com/jbellister-slac/pydm/actions/runs/5449701997

The PyPI job was tested here against the test PyPI repository:
https://github.com/jbellister-slac/pydm/actions/runs/5470029942

[YektaY]

This all looks great!