Investigate possible refactors after deprecating Python 3.8

[georgesittas]

Support for Python 3.8 was dropped in 7a7a9f6, which means there may be an opportunity to refactor a few places:

1. d6f3c7c: add custom source parser because it's broken in < 3.9 – simplify the definition of _is_relative_to [1]
2. 92242e2: allow python env building to use relative paths – simplify the python source parser
3. b4dbb45: Fix: Remove intervals instead missing intervals – address the TODO in dag.py

[1]: we may not be able to get rid of this completely, because it's been extended since the first implementation.