build: Add support for NumPy 2.0 wheels

[matthewfeickert]

Resolves #22

• Add numpy>=2.0.0 to build-system requires to build NumPy 1.x and 2.x compatible wheels.
  • c.f. https://numpy.org/doc/stable/dev/depending_on_numpy.html#numpy-2-0-specific-advice
  • As NumPy 2.0 is Python 3.9+, also need to conditionally support 'oldest-supported-numpy' for Python 3.8 and older.
• Remove upper bound on numpy version as POT v0.9.4 supports NumPy v2.0 wheels.
  • c.f. https://github.com/PythonOT/POT/releases/tag/0.9.4

• Use NumPy 2.0 copy for arrays via asarray

  • Use numpy.asarray as already dealing with array-like data.

  • Remove the copy kwarg as for NumPy 2.0 the default is None which makes a copy only if needed:

    If None then the object is copied only if needed, i.e. if array returns a copy, if obj is a nested sequence, or if a copy is needed to satisfy any of the other requirements (dtype, order, etc.).

    In NumPy 1.26 numpy.asarray, NumPy will try to avoid a copy if possible

    No copy is performed if the input is already an ndarray with matching dtype and order.

[matthewfeickert]

@j-s-ashley This is ready for review.