- commit 4f7bca1931 replaced orderedset by the maintained ordered-set,
but did no yet remove the dependency. Since on more recent python
version the dependency install now fails, we are removing it completely.
- make python 3 requirement explicit
To be released on PyPi simultaneously with the next CAmkES release.
Closes#124
Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>