r202639: typed rate-limit error on registration

[cayossarian]

r202639: a typed error for rate-limited registration. Ports Don's (@dcj) acceptance tests for registration rate limits onto this line unchanged: tests/test_register_rate_limit.py. Thanks, Don.

• register_v2 raises the new SpanPanelRateLimitError (a SpanPanelAPIError, not an auth error) on HTTP 429 and does not retry. retry_after_s is the wait the panel asked for, in seconds: Retry-After delta-seconds as given, an HTTP-date as the seconds until then; None when absent, negative, infinite or neither form. The body and the passphrase stay out of the message.
• The CA download's 429 backoff reads Retry-After through the same parser, so it now takes an HTTP-date and no longer waits on an infinite value.
• The passphrase rotation docstring names a 504, beside a 500, as an unknown outcome.
• tests/expected_failures.py lists two of Don's tests as divergent: they read the delay as retry_after and expect None for an HTTP-date. A strict xfail stops at its first failing line, so tests/test_register_rate_limit_companions.py pins the delay in seconds and the statuses around 429. Don's file is never edited. The registry now also finds methods of test classes.

Registry rows

Test	Kind	Flips when
test_raises_the_rate_limit_class_with_its_status	divergent	stays: the delay is retry_after_s, in seconds
test_retry_after_is_none_without_a_usable_header (all cases)	divergent	stays: an HTTP-date Retry-After reads as the seconds until then

Checks

• pytest tests -q: 1520 passed, 7 skipped, 27 xfailed (before: 1504 passed, 7 skipped, 21 xfailed); each commit passes on its own
• pre-commit run --all-files: all hooks pass, mypy included
• Merged with the current prerelease/3.8 locally (no conflicts, not committed): 1639 passed, 7 skipped, 20 xfailed