stepss v3.81.1

Manual release of the python package. Neither bundled library changed;
the versions below are the ones this release inherits from the previous one.

Component	Version
RAMSES	v3.81
Helios C API	v1.4.1

The wheel attached here is the exact artifact the regression gate ran
against on Linux, Windows and macOS: the full test suite, including the
Nordic voltage-collapse trajectory check, passed on all three before this
release was cut. It is this file, not a rebuild, that the automation
then uploads to PyPI.