bpo-43295: Fix error handling of datetime.strptime format string '%z' (GH-24627)...

bpo-43295: Fix error handling of datetime.strptime format string '%z' (GH-24627) (#25695)

Previously, `datetime.strptime` would match `'z'` with the format string `'%z'` (for UTC offsets), throwing an `IndexError` by erroneously trying to parse `'z'` as a timestamp. As a special case, `'%z'` matches the string `'Z'` which is equivalent to the offset `'+00:00'`, however this behavior is not defined for lowercase `'z'`.

This change ensures a `ValueError` is thrown when encountering the original example, as follows:

```
>>> from datetime import datetime
>>> datetime.strptime('z', '%z')
ValueError: time data 'z' does not match format '%z'
```

Automerge-Triggered-By: GH:pganssle
(cherry picked from commit 04f6fbb6969e9860783b9ab4dc24b6fe3c6dab8d)

Co-authored-by: Noor Michael <nsmichael31@gmail.com>
Co-authored-by: Noor Michael <nsmichael31@gmail.com>