BUG: fix Irap ascii export bug that may occur in some cases

BUG: fix Irap ascii export bug that may occur in some cases

ENH: add attributes for Polygons when reading RMS

BUG: re-activate yflip on Irap export

CLN: move towards structs in C++/pybind11

BLD: Publish for macOS Python 3.9

BLD: skip support for python 3.8

BUG: undef values in grdecl export shall be 0 or 1

BLD: Pin swig<4.3.0

This version and the changes to numpy.i that come with it (pulled
directly from the numpy repository) caused segfaults under certain
circumstances in Python 3.8, in particular when in a context where
pandas>2 is used. It's very difficult to understand the interactions
causing these problems and the most prudent thing to do seems to be to
wait until we deprecate support for Python 3.8 and then unpin swig.

CI: Revert macOS runner pkg-config work-around

GitHub has updated their macOS images to install pkgconf over
pkg-config.