Add support for one-digit month

[vapaj]

Description of the change Adds support for one-digit months, e.g. "20.5.2020". This is pretty lenient, as "20.05.2020" will format nicely too 😁

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Checklist:

• [x] Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• [ ] Linked any existing issues or proposals that this pull request should close
• [ ] Updated or added relevant documentation in the README and/or documentation directory
• [ ] Added a test for the contribution (if applicable)