Address warnings

[sjperkins]

• Tests added / passed

  $ py.test -v -s africanus

  If the pre-commit tests fail, install and
  run the pre-commit hooks in your development
  virtuale environment:

  $ pip install pre-commit
  $ pre-commit install
  $ pre-commit run -a

• Fully documented, including HISTORY.rst for all changes
  and one of the docs/*-api.rst files for new API

  To build the docs locally:

  pip install -r requirements.readthedocs.txt
  cd docs
  READTHEDOCS=True make html

[sjperkins]

@bennahugo Just letting you know that the test cases you wrote easily highlighted up the above needed change, due to the following https://numba.readthedocs.io/en/stable/reference/deprecation.html#deprecation-of-old-style-numba-captured-errors.

So the test cases were useful to have and I didn't have to understand the implementation to do the upgrade.