split 'vunit/ui.py' into a submodule/package 'vunit/ui/*' (VUnit#572)

* refactor: split 'ui.py' into multiple 'ui_*.py' files
* refactor: split/move 'ui' to submodule
* ui: split 'VUnit' to ui/vunit.py
* ui: python2 requires relative imports
* docs: create subdir 'py', move content from py sources, add headers/subsections

.github/workflows/registry.yml

@@ -0,0 +1,38 @@
+name: 'registry'
+
+on:
+  push:
+    paths:
+      - '.github/workflows/registry.yml'
+
+env:
+  DOCKER_BUILDKIT: '1'
+
+jobs:
+  docker:
+    strategy:
+      fail-fast: false
+      max-parallel: 1
+      matrix:
+        registry: [
+          docker.pkg.github.com/VUnit/,
+          docker.pkg.github.com/vunit/
+        ]
+        repo: vunit/dev
+    runs-on: ubuntu-latest
+    steps:
+    - run: docker pull alpine
+    - name: docker login
+      env:
+        DOCKER_USER: ${{ secrets.DOCKER_USER }}
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      run: |
+        echo "$GITHUB_TOKEN" | docker login -u "$DOCKER_USER" --password-stdin docker.pkg.github.com
+    - name: deploy
+      env:
+        IMG: ${{ matrix.registry }}${{ matrix.repo }}:alpine
+      run: |
+        docker tag alpine "$IMG"
+        docker push "$IMG"
+    - run: docker logout
+      if: always()