Add opt-in transport retries following PayWay retry guidance

[napper1]

Summary

Adds opt-in transport retries to Client, following PayWay's retry guidance:

• New keyword-only constructor kwargs: Client(..., max_retries=0, retry_delay=1.0). Default max_retries=0 keeps existing single-attempt behaviour — fully backwards compatible.
• Retries on requests.ConnectionError, requests.Timeout, and HTTP 429/503, with linear backoff (retry_delay × attempt), honouring a numeric Retry-After header when present. retry_delay is configurable since synchronous checkout flows can't afford PayWay's suggested 20s wait.
• Never retries other status codes (including 500/502/504, per PayWay's advice) or declined transactions.
• POSTs are only retried when an idempotency_key was supplied, and the same Idempotency-Key header is resent on every attempt so PayWay replays the original response instead of double-charging. PUTs never carry a key, so they never retry. GETs always retry.
• After retries are exhausted, behaviour is identical to a single failed attempt today — the (response, errors) contract is unchanged.

Also documents the new params in the README, adds a CHANGELOG, and bumps the version to 0.0.8.

Test plan

• New TestClientRetries unit tests (HTTP layer mocked): connection error then success (same key on both attempts), 429 then success (waits, honours Retry-After), 503 exhausting retries, 500 not retried, POST without key not retried, default max_retries=0 single attempt, GET retried.
• Full suite: 27 passed. ruff check payway and mypy payway clean.

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