Added publishing workflow

[patrick-kidger]

Prompted by (and I expect superseding) #100, I've put together a publish workflow. It should automatically upload a build to PyPI whenever we do a release.

Untested for obvious reasons.

At the moment it uses a PyPI username + password for authentication. We can change that to using an API token if you prefer.

[patrick-kidger]

(We should create a v0.2.5 release before merging this.)

[lxuechen]

(We should create a v0.2.5 release before merging this.)

We could tag the commit that comes from merging #102. Thanks again for putting all of this together!