Status of Python branches

[eine]

This issue is just a quick reference to the official release schedules of Python versions:

• https://devguide.python.org/devcycle/#end-of-life-branches
• https://devguide.python.org/#status-of-python-branches

In the CI pipelines, we are currently testing the latest and newest supported Python 3 versions: 3.5 and 3.7/3.8 (depending on the service/context). We are also testing Python 2.7 until next month (see #596) Python 3.6, 3.9 and 3.10.