Missing complex value support for SciPy special functions in Numba

[brandonwillard]

It looks like Numba's math.erf does not support complex numbers, which means that our transpilations won't either.
For example:

import math
import numba as nb
import scipy


@nb.njit
def math_erf_fn(x):
    return math.erf(x)


math_erf_fn(1.0)
# 0.8427007929497149

math_erf_fn(complex(1.0))
# TypingError: Failed in nopython mode pipeline (step: nopython frontend)
# No implementation of function Function(<built-in function erf>) found for signature:
#
#  >>> erf(complex128)


@nb.njit
def scipy_erf_fn(x):
    return scipy.special.erf(x)


scipy_erf_fn(1.0)
# 0.8427007929497148

scipy_erf_fn(complex(1.0))
# TypingError: Failed in nopython mode pipeline (step: nopython frontend)
# No implementation of function Function(<ufunc 'erf'>) found for signature:
#
#  >>> erf(complex128)

This was observed via tests.tensor.test_math_scipy failures in #1289.