Drop support for Python 3.6

[redeboer]

It is becoming increasingly bothersome to support Python 3.6. This issue keeps track of arguments for dropping support:

• Postponed evaluation of annotations (PEP 563)
• Simplified union type notation (if using from __future__ import annotations).
• https://github.com/ComPWA/qrules/pull/156#issuecomment-1046853982
• https://github.com/zfit/phasespace/pull/69#issuecomment-977952982
• ComPWA/repo-maintenance#59
• SymPy is not tested on Python 3.6 anymore. See SymPy v1.10 and SymPy version support policy.

Useful tools

• pyupgrade
• flake8-future-annotations

See also

• What’s New In Python 3.7
• Scikit-HEP's Statement on Python 3.6

### Progress
- [ ] QRules
- [ ] https://github.com/ComPWA/qrules/pull/250
- [x] https://github.com/ComPWA/ampform/pull/249
- [x] https://github.com/ComPWA/tensorwaves/pull/431
- [x] https://github.com/ComPWA/compwa-org/pull/132
- [x] https://github.com/ComPWA/PWA-pages/pull/189
- [ ] https://github.com/ComPWA/repo-maintenance/pull/234

[redeboer]

Group meeting decision 25.02.2022: drop support once new problems appear.