Migrate versioning to CalVer

[bachya]

Describe what the PR does:

After much consideration, I've decided I want to move from Semantic Versioning ("SemVer") to Calendar Versioning ("CalVer") so that I can avoid the typical issues associated with SemVer (like a number indicating a more "mature" library than what exists).

In general, the library will now follow a version format of YEAR.MONTH.PATCH. There is no special significance to the 0 patch; instead, users should understand these examples:

• YEAR.MONTH.0 is merely the first release of that month (regardless of where in the month it happens).
• YEAR.MONTH.3 is the third release of the month.

This PR focuses on updating the release script.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md with any new documentation.
• Add yourself to AUTHORS.md.

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