ci: Publish releases to PyPI through GitHub releases

[matthewfeickert]

• Resolves Preferences for CD workflow to publish to PyPI #36
• Resolves make pandamonium this pip-installable #26

This PR uses publication type releases to publish to PyPI by running the publication workflow on published release events and then verifying that the PyPI upload step is running on one. This follows what I'll call the corner model (which I now think is how GitHub Actions assumed people would do things):

This follows the example that is setup in corner with running the workflow on published release events and then only running the publish to PyPI step on that conditions. To instigate this workflow one would do the following:

• Locally on master run:

$ git checkout master && git pull # verify that you're on master and synced with GitHub
$ bump2version <part> # bump the version and create a commit and tag
$ git push origin master --tags # push the commit and the tag to GitHub, causing TestPyPI to publish

• Then on GitHub:
  1. Go to releases: https://github.com/dguest/pandamonium/releases
  2. Click "Draft a new release"
  3. On the new page enter the tag you just pushed (e.g. v0.2.0) in the "Tag version" box and the "Release title" box (to make it easy unless you really want to get descriptive)
  4. Enter any release notes and click "Publish release"
• This then kicks of the publication CD workflow that will use the PyPI API key to publish.

This is a bit different from the pyhf way of doing things, but also lets things run through on TestPyPI on the tag before things are ever released.

Additionally, this PR adds support for manual triggers of workflows through workflow dispatch

Recommended squash and merge commit message:

* Publish to PyPI through published release events triggered through GitHub releases
   - c.f. https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows#release
* Add workflow dispatch to all GitHub Action workflows

[matthewfeickert]

@dguest This is one possible solution to Issue #36. Let me know your thoughts.

[matthewfeickert]

@dguest Given that GitHub Actions is deprecating set-env and add-path commands the pyhf way of publishing is going to have to change so it seems that the method I'm implementing here is the best way forward for the time being.