drop cython language_level on v3.0.0

[bjlittle]

📰 Custom Issue

Once cython 3.0.0 lands:

• drop use of the cython language_level compiler directive (3str will be the default behaviour)
  • explicitly set cython language_level #358
• add a min pin for cython>=3.0 for conda and PyPI deps
• address warning: #warning "Using deprecated NumPy API, disable it with " "#define NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION" [-Wcpp], see
  • https://cython.readthedocs.io/en/latest/src/userguide/migrating_to_cy30.html?highlight=NPY_NO_DEPRECATED_API#numpy-c-api
  • https://cython.readthedocs.io/en/latest/src/tutorial/numpy.html?highlight=import_array#adding-types

See https://github.com/cython/cython/milestone/58