Reject out-of-range UTC offsets in the pure-Python parsers - #1015

1. The ISO 8601 parser (parse_iso8601) and from_format both build a timezone offset from the parsed hour and minute without bounding its magnitude, so an offset like +99:59 is accepted.
2. That yields a FixedTimezone whose utcoffset sits far outside the plus/minus 24h datetime permits, and the returned value raises the moment it is used (isoformat, astimezone, comparison across zones).

Added an upper bound in both callees, matching the Rust parser which already rejects offsets above 24 hours.

• Added tests for changed code.
• Updated documentation for changed code.