Ipywidgets-extended

Update dependencies - stop supporting ipywidgets v7.x

v1.2.x will be the last release to support ipywidgets v7.x as v8 has been released.

Drop Python 3.6 support.
Several minor fixes.

**Updates**:
- Tweak grouping flattening methods (46)

**Developer updates**:
- Use vMAJOR GH Actions where appliccable (44)
- Update pre-commit hooks (20, 28, 32, 37, 44)
- Update development dependencies (20, 28, 32, 39, 40, 44):
- `invoke~=1.6`
- `pre-commit~=2.15`
- `pylint~=2.11`

**Updated**:
- **New grouping feature for `DropdownExtended` widget (16)**.
By using the `grouping` parameter, one can group options with headers within the dropdown.
For more information see the example notebook.

**Developer updates**:
- Fixed dependabot target branch (15).
- Update dependencies according to dependabot (15):
- `ipywidgets` (~=7.6)
- `invoke` (~=1.5)
- `pylint` (~=2.7)
- `pre-commit` (~=2.11)
- Update GitHub Action actions according to dependabot (15):
- `actions/setup-node` (v2)
- `actions/cache` (v2.1.4)
- Update pre-commit hooks (15).

**Changes**:
- Publish also the `wheel` for the package (OS: Linux/Ubuntu) ensuring installability in more "thin" environments (7).

**Fixes**:
- Ensure distribution properly installs the Jupyter notebook extension (6).

**Developer updates**:
- Check if the installation successfully installed the Jupyter notebook extension in the CI after building the source distribution and installing it (6).