Kwant 1.4.1 is required because 1.4.0 does not accept value functions
that take *args.

tkwant.leads exposes functions/classes that take a 'params' argument.
Setting this to anything else than the default 'None' currently does not
work with Kwant 1.4 (it should continue to work with Kwant 1.3).  This
problem will be resolved once tkwant is rewritten to no longer use
Kwant's args mechanism internally.