Fix UTCDateTime Regex for ordinal day 360

[calum-chamberlain]

What does this PR do?

Adapts the _ISO8601_REGEX to catch ordinal day 360.

Why was it initiated? Any relevant Issues?

This patch copes with issue #2868

PR Checklist

• Correct base branch selected? master for new features, maintenance_... for bug fixes
• This PR is not directly related to an existing issue (which has no PR yet).
• If the PR is making changes to documentation, docs pages can be built automatically.
  Just remove the space in the following string after the + sign: "+ DOCS"
• If any network modules should be tested for the PR, add them as a comma separated list
  (e.g. clients.fdsn,clients.arclink) after the colon in the following magic string: "+TESTS:"
  (you can also add "ALL" to just simply run all tests across all modules)
• All tests still pass.
• Any new features or fixed regressions are be covered via new tests.
• Any new or changed features have are fully documented.
• Significant changes have been added to CHANGELOG.txt .
  - [ ] First time contributors have added your name to CONTRIBUTORS.txt .

[ThomasLecocq]

@d-chambers how do you cherry-pick maintenance items like this one back to master, as it's an important one ?

[ThomasLecocq]

I cherry-picked in pycharm & pushed back to master (not sure it's the recommended way ...)