Update build system to support Python 3.12 and f2py using meson

[elcorto]

Starting with Python 3.12, f2py enforces using meson instead of the deprecated distutils as the backend. This breaks our f2py call in the Makefile.

The build of _flib fails

gfortran -I_flib.cpython-312-x86_64-linux-gnu.so.p -I. -I.. -I/usr/lib/python3/dist-packages/numpy/core/include -I/usr/lib/python3/dist-packages/numpy/f2py/src -I/usr/include/python3.12 -I/usr/include/x86_64-linux-gnu/python3.12 -fvisibility=hidden -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -O3 -fPIC -J_flib.cpython-312-x86_64-linux-gnu.so.p -o _flib.cpython-312-x86_64-linux-gnu.so.p/flib.f90.o -c ../flib.f90
../flib.f90:152:2:

  152 | #define stdout 6
      |  1
Warning: Illegal preprocessor directive

[...]

../flib.f90:224:24:

  224 |             write(stderr,*) this, "ERROR: illegal value for 'use_m'"
      |                        1
Error: Symbol ‘stderr’ at (1) has no IMPLICIT type

It looks like none of our compiler options passed to f2py, including F90FLAGS, are getting through, in particular -x f95-cpp-input to handle preprocessor statements.

Seems like a quick fix is to adopt the new(?) CLI interface to pass those options (see https://numpy.org/doc/stable/f2py/buildtools/distutils-to-meson.html). For Python < 3.11, we'd need to use f2py ... --backend meson then.

Maybe take this opportunity to check of we can replace the custom setup.py + Makefile workflow altogether.