Readthedocs configuration file changes #252

Unfortunately readthedocs changed the format of the configuration file .readthedocs.yml. Therefore our branches 3.7-slp and 3.6-slp fail to build. See readthedocs/readthedocs.org#8204.

We must update the our configuration file .readthedocs.yml. See https://docs.readthedocs.io/en/latest/config-file/v2.html. For 3.7 and 3.6 I'll set the build image to 'stable' and the python version to 3.7 respective 3.6.

In order to build the documentation readthedocs must see the configuration file. Therefore we must either move the existing tags or add new tags. Because the file .readthedocs.yml is not part of our source code archive, I'll simply recreate the tags v3.7.9-slp and v3.6.13-slp.