Cython support

NumPy's Cython headers (accessed via cimport numpy) now require Cython 3.0 or newer to build. If you try to compile a project that depends on NumPy's Cython headers using Cython 0.29 or older, you will see a message like this:

Error compiling Cython file:
------------------------------------------------------------
...
# versions.
#
# See __init__.cython-30.pxd for the real Cython header
#

DEF err = int('Build aborted: the NumPy Cython headers require Cython 3.0.0 or newer.')
  ------------------------------------------------------------

  /path/to/site-packages/numpy/__init__.pxd:11:13: Error in compile-time expression: ValueError: invalid literal for int() with base 10: 'Build aborted: the NumPy Cython headers require Cython 3.0.0 or newer.'

Note that the invalid integer is not a bug in NumPy - we are intentionally generating this error to avoid triggering a more obscure error later in the build when an older Cython version tries to use a Cython feature that was not available in the old Cython version.