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.

pyproject.toml

@@ -2,7 +2,7 @@
 requires = [
     "pybind11",
     "scikit-build-core[pyproject]>=0.10",
-    "swig",
+    "swig<4.3.0",
     "numpy==1.19.2; python_version == '3.8'",
     "numpy==1.19.5; python_version == '3.9'",
     "numpy==1.21.6; python_version == '3.10'",