can we drop cython as a dependency in favor of numba

[sergpolly]

In the light of this #467 - and there was something else about not being able to switch to hatch because of Cython - can we complete the rewrite of numutils.pyx to numutils.py with all things numba ?

Brief comparison between numuils.pyx and numutils.py:

# functions defined in numutils.pyx
logbins
observed_over_expected
iterative_correction_symmetric
fake_cis
_matvec_sparse_symmetric
_matvec_product
# + one more class definition 
MatVec(object)

Related or matching functions defined in numutils.py:

# similar to logbins from .pyxc
_logbins_numba
persistent_log_bins
# O/E used in eigdecomp
observed_over_expected
# ICE-ing
iterative_correction_symmetric
iterative_correction_asymmetric

# looks like fake_cis and MatVec-related ones
# are the only ones that's missing !?

Superficially i conclude from that that we need to re-implement fake_cis and something like MatVec using numba - and then deprecate whole cython thing as a dependecy.
PS
At least as long as my assumption that numba is easier to package and is similarly fast -> is correct