Use qtpy, deprecate superqt.qtcompat, drop support for Qt <5.12 (#39)

* remove qtcompat

* change imports

* change codecov

* add dep

* run tests against dalthviz branch

* update

* more replace

* pin pyside6 for sake of test

* add deprecation

* drop qt 5.11

* unpin pyside6

.github/workflows/test_and_deploy.yml

@@ -75,12 +75,6 @@ jobs:
             backend: pyqt5
 
           # legacy Qt
-          - python-version: 3.7
-            platform: ubuntu-latest
-            backend: pyside511
-          - python-version: 3.7
-            platform: ubuntu-latest
-            backend: pyqt511
           - python-version: 3.7
             platform: ubuntu-latest
             backend: pyqt512