pytype is grumpy about a sub method having a different signature than the one we
use here.

pytype error:

  internalmerge: Overriding method signature mismatch [signature-mismatch]
    Base signature: 'def _funcregistrarbase._extrasetup(self, name, func) -> Any'.
    Subclass signature: 'def internalmerge._extrasetup(self, name, func, mergetype, onfailure = None, precheck = None, binary = False, symlink = False) -> Any'.
    Parameter 'mergetype' must have a default value.