REL: Prepare for the NumPy 1.18.1 release.

- Update release notes.
- Create changelog.

[ci skip]

MAINT: Do not use private Python function in testing

BUG: lib: Fix handling of integer arrays by gradient.

MAINT: only add --std=c99 where needed

In numpy.gradient, convert integer array inputs to float64 to avoid
unwanted modular arithmetic.

Closes gh-15207.