This bug was caused by wraparound abusing a builder to hold a _list_ of values.
Builders do know how to handle Hermiticity for values that are scalars and
functions, but not for lists.

It was the original sign convention that was correct.  The mistake occurred
because I had a stale Kwant 1.0 installed in /usr/local (dating from some pip
experiment it seems) that uses the opposite convention as current Kwant.

This reverts commit 50d832ea.