Make pint dependency really optional (py313 compat)

[getzze]

pint is not compatible with python 3.13 and it may take a while to make it compatible.

pint is a hard dependency for knowit although it is imported in a try..except structure in case it is not installed.

This PR makes two changes:

• deal with the case of having pint installed but broken (so knowit would work with py313 now and in the future if pint breaks).
• make pint an optional dependency. To install pint when installing knowit, you would need to use:
  pip install knowit[pint]

On a side note, maybe enzyme and pymediainfo could be made optional dependencies too, as they are not strictly needed for knowit to work.