Support short UTC offset timezones

[CodingFeng101]

Summary

Support two-digit UTC offset timezone strings such as +08 and -03 when they are passed through timezone settings. These values are produced by Python's timezone APIs on some systems, but currently raise UnknownTimeZoneError while equivalent forms like UTC+08, +0800, and +08:00 are accepted.

Changes

• Normalize +HH / -HH timezone strings to the existing UTC+HH matching path.
• Add focused regression coverage for positive and negative short UTC offsets.

Validation

• python -m pytest tests/test_short_utc_offsets.py -q -> 2 passed
• Manual smoke check confirmed dateparser.parse(..., settings={"TIMEZONE": "+08"}) and "-03" no longer raise.