Remaps minecraft mod class names from one version to another.
py remapper.py apply <target> <mappings>
where target is the file or directory you want to remap, and mappings is the mapping csv you want to use.
This outputs to an /output directory your code but with all references to the old classes changed to the new ones.
Merging Mappings:
You can merge mappings between two version by using merge as the first argument:
py remapper.py merge 1.12-to-1.13.1.csv 1.13.1-to-1.14.3.csv > 1.12-to-1.14.3.csv