FastDTW fails to compile due to invalid fallback `from numpy.math cimport INFINITY`

[QuirkyCroissant]

When building fastdtw from source (e.g., via python setup.py build_ext --inplace), the build fails with:

'numpy/math.pxd' not found

This is due to a fallback block in _fastdtw.pyx:

try:
    from libc.math cimport INFINITY
except:
    from numpy.math cimport INFINITY  # this file doesn't exist

Proposed fix: If I have not done something completely wrong, I replaced the block with the correct C import directly:

from libc.math cimport INFINITY

This resolves the build issue and enables the actual fast C extension to be used.
Let me know if you'd like me to submit a PR - happy to help.