Update the jobs to use the correct python version, and drop python 3.6

[prusse-martin]

A part of the log for the "build (39)" (from #165)...

============================= test session starts ==============================
platform linux -- Python 3.6.15, pytest-6.2.5, py-1.11.0, pluggy-1.0.0 -- /usr/share/miniconda/envs/qmxgraph/bin/python
cachedir: .pytest_cache
PyQt5 5.12.3 -- Qt runtime 5.12.9 -- Qt compiled 5.12.9
metadata: {'Python': '3.6.15', 'Platform': 'Linux-5.15.0-1042-azure-x86_64-with-debian-bookworm-sid', 'Packages': {'pytest': '6.2.5', 'py': '1.11.0', 'pluggy': '1.0.0'}, 'Plugins': {'qt': '4.0.2', 'variables': '1.7.1', 'mock': '3.6.1', 'timeout': '2.1.0', 'xvfb': '2.0.0', 'rerunfailures': '10.3', 'hypothesis': '3.11.0', 'base-url': '1.4.1', 'metadata': '1.11.0', 'selenium': '1.17.0', 'html': '3.2.0', 'cov': '2.5.1'}, 'CI': 'true', 'JAVA_HOME': '/usr/lib/jvm/temurin-11-jdk-amd64', 'Base URL': '', 'Driver': 'PhantomJS', 'Capabilities': {}}

Note that it is running using python 3.6 and looks like most libraries are dropping support for it.