Add dependabot

[griff-rees]

dependabot can help manage updating dependencies, including and potential security updates.

[griff-rees]

Need to address failures in pull requests #58 and #57, but keys are needed for depdendabot pull requests that are separate from existing repository keys.

[griff-rees]

Added dependabot keys for both PyPI and TestPyPI via a new living-with-machines account for both:

• https://pypi.org/user/living-with-machines/
• https://test.pypi.org/user/living-with-machines/