NumPy build utils does not know what to do with .f03 and .f08 files. This makes it difficult to compile a library written in Fortran 2003 or 2008.

Fortran 2003 is e.g. useful if we want to make a C callable library without depending on an ABI wrapper. A typical usecase is calling Fortran from Cython, which is more flexible and efficient than using f2py.

f2py does not parse Fortran 2003. But that is not a problem if we just want to use configuration.add_library, and not configuration.add_extension (which will invoke f2py).