Automated version bumping and pypi package building

[samwinebrake]

This PR adds automated version bumping capabilities to the repo. Now whenever a PR is merged that isn't plugin-only, it will automatically trigger a version-bumping pipeline.

The current versioning system is v.major.minor.patch; merging a PR with no update label will cause the patch version to be updated, adding the minor update label will increment the minor version, and adding the major update label will increment the major version.

After the version is bumped, 2 different jobs run:

1. Release notes are generated that outline what has changed, and identify the contributor(s) of the changes.
2. The updated package is built and published to PyPI.

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NOTE: Everything has been tested locally and on a fork, with the exception of the final publishing step to PyPI which will be tested on the merging of this PR.

[KartikP]

LGTM, will this workflow need to be implemented in the other repos?

[mike-ferguson]

Also LGTM, quick question: what is the default tag for a non plugin-only PR now? minor? As in any change that is not just a plugin will be a minor, unless otherwise specified to be major via the major tag?

[samwinebrake]

Also LGTM, quick question: what is the default tag for a non plugin-only PR now? minor? As in any change that is not just a plugin will be a minor, unless otherwise specified to be major via the major tag?

@mike-ferguson All PRs without a minor or major tag will be just added as a patch!

[samwinebrake]

LGTM, will this workflow need to be implemented in the other repos?

Yes, after this is PRed will implement something similar in core!