Upgrade isodate library

[ageron]

Upgrade the isodate library to 0.5.1, which supports finding the weekdays and day of month of a given date. This will come in handy for the meetup exercise.
Note: only merge this PR once the roc-test-runner's cache includes this upgraded library (i.e., when exercism/roc-test-runner#16 is merged and deployed).

This upgrade does not affect the existing user solutions, so we must include [no important files changed] in the merge-commit message.

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

For more information, refer to the documentation. If you are unsure whether to add the message or not, please ping @exercism/maintainers-admin in a comment. Thank you!