Fix test logic for datetime

[ageorgou]

Summary of changes

One of the assertions in test_datetime_z seems to be performing operations in the wrong order. This also means it's passing trivially, as it's essentially recreating the expected result string and asserting it's not empty.

This PR changes the test to what I think was the intended order of operations:

• format the parsed datetime value (l.toPython()) according to the given format string
• compare against the expected result string
• assert they're equal

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

coverage: 90.637%. remained the same
when pulling a152494 on ageorgou:fix-date-test
into 2f2cf94 on RDFLib:main.