fix: preserve HTTP error subclasses when pickling

[Shubham-Padkonde]

HTTPError.__reduce__ always reconstructs the base HTTPError class. Consequently, pickling a TooManyRequestsError or ServiceUnavailableError succeeds but loses its type. Callers receiving an exception from a worker can no longer catch the original subclass to apply their retry policy.

Reconstruct type(self) with the existing four serialized fields. This follows up on #148 and retains its pickle support while preserving the exception class.

The regression covers every built-in status-specific error and the generic fallback, using every pickle protocol available in Python 3.12. All 66 subclass/protocol combinations fail before the fix and pass afterward; all 12 client unit tests pass. Pycodestyle and Pyflakes pass on both changed files.

Full discovery runs 14 tests: 13 pass and the existing copyright-year test fails because LICENSE says 2023 while the current year is 2026. No license text was changed. The legacy Python versions in CI were not run locally.

Checklist

• Contributions are made under the project's license.
• This is a functional change with regression coverage.
• Read the contribution guidelines; this targets main, the current CI/default branch (development no longer exists).
• The PR uses a conventional title.
• Tests demonstrate the fix.
• No new public API or documentation changes are required.