[Config] Use one global config

[benedikt-voelkel]

A global config key 'config' can be used to pass a dictionary which will be available everywhere.

Per step, it can be extended or changed by passing a dict in that stage, also using the key 'config'

[benedikt-voelkel]

This simplifies the usage of the global configuration. Before, it would have been required to copy the full config into each individual config of a stage.

[benedikt-voelkel]

d = d1 | d2

for dictionaries only works from python 3.9 onwards... Maybe 3.8 is simply too old to be worth testing anymore?!

[mconcas]

Yep, python 3.8 is eol since 4y+. I'd remove it from matrix test.

[benedikt-voelkel]

Ok, this one works now as well.
Next one will be an update for the README