CICD: Release PR Creation Workflow and Slack Messaging

[algobarb]

Summary

• Adds a Release PR Creation github workflow
• Add Slack Messaging after a release branch completes CircleCI tests
• Add a script/bump_version.py to handle bumping version in setup.py
• Support Hot-Fixes

Testing

• The Description is populated with the changelog as expected in this PR: FOR REVIEW ONLY: py-algorand-sdk v0.0.0 #495
• Slack message can be found in our internal #devops-experimental-notifications slack channel.

[algobarb]

Closing to test adding pre-release steps. Will re-open PR afterwards.

[algobarb]

Thanks @excalq for the pointers!

I added the recommended changes as well as a few more changes in the rest of the yml file to remove the input parameters: release_branch_exists and source_ref:

• I added fetch-option: 0 to the checkout step so it will fetch all of the branches rather than just one commit, so we can check if the release branch exists on remote.
• I also added a condition that if the release branch exists, it will error if we aren't running the workflow from the release branch. It could indicate that someone may not have realized the release branch was already created.
• I removed source_ref parameter and instead will use the "workflow_ref", i.e. the ref used to run the workflow.

[excalq]

That's a good validation check.

[excalq]

Awesomesauce.