Use bumpversion to manage versions across build systems

[lunacd]

Using a tool like bumpversion can keep this kind of special wiring logic to a minimum, but no need to hold up this PR on that. Someone can circle back to that in the future if it's interesting.

Originally posted by @bretbrownjr in #82 (comment)

[bretbrownjr]

If we need to hardcode our version in multiple places, we could use CI time hooks to make sure we're consistent about it. bumpversion is one option, but I'm actually flexible on the specifics.