fix: retry uploads on a 408 request_timeout response (#605)

[ffumero2003]

Problem

An upload that receives a 408 request_timeout from B2 raises UnknownError to
the caller after a single attempt — no retry, no fresh upload URL — unlike the
401 (expired token), 500, and 503 cases, which all retry. Backblaze's Integration
Checklist lists 408 as a recognized upload failure after which the client should
get a new upload URL and retry.

Root cause

interpret_b2_error has an explicit branch for every status it treats specially
(400s, 401, 403, 409, 416, 429, 500–599) but none for 408, so a 408 falls through
to return UnknownError(...). UnknownError is a bare B2SimpleError and does
not mix in TransientErrorMixin, so should_retry_upload() is False — and
UploadManager's retry loop gives up immediately.

Fix

Map 408 to a retryable exception, mirroring how 500/503 map to ServiceError:

• Add RequestTimeout(TransientErrorMixin, B2Error) (so should_retry_upload()
  and should_retry_http() are both True).
• Add elif status == 408: return RequestTimeout(...) to interpret_b2_error,
  before the 429 branch.

Scope is limited to the exception mapping; UploadManager and the transport
retry wiring are unchanged — the retry gate was already correct, it just never
received a retryable exception for 408.

Testing

• New unit test asserting interpret_b2_error(408, ...) returns a RequestTimeout
  whose should_retry_upload() and should_retry_http() are both True
  (red before the change, green after).
• Full unit suite passes across all API versions (nox -s unit, apiver v0–v3).

Fixes #605.

[sophiecarreras]

This is superseded by #618, which carries the same change (byte-identical diff for the code, tests and changelog fragment). Closing this one avoids two PRs adding the same fragment and test. Please close whichever you want to drop; I suggest keeping #618.

[ffumero2003]

Superseded by #618, which merged to master with the same fix plus the review nits (safe Retry-After parsing / v3 export etc.). The change here is already on master, so closing this duplicate.