test(specifiers): stop filtering compatible-release examples by epoch

[cpruijsen]

Fixes #1406.

TestCompatibleReleaseDefinition.test_compatible_release_matches_expected_compatible sets the independently drawn candidate's epoch to the specifier's with Version.__replace__ instead of assume(spec_ver.epoch == candidate.epoch). Independent draws often differed in epoch, so Hypothesis sometimes filtered 50 examples before accepting 10 and failed HealthCheck.filter_too_much. Same-epoch coverage is kept, and examples are no longer rejected.

Decision

Align epoch on the candidate after generation. Alternatives: generate a shared-epoch pair (open #1407 adds an epoch= argument on pep440_versions plus a pair strategy), drop the epoch constraint, or suppress HealthCheck.filter_too_much. This file already derives related versions with __replace__, and sibling ~= tests already cover mixed-epoch candidates against the expanded form. Suppressing the health check would leave the filtering in place.

Can switch to a composite strategy or to allow mixed epochs.

Test plan

• Health check still fires on this tree with --hypothesis-seed=198
  before the change
• Same seed passes after the change
• python -m pytest -m property tests/property/test_specifier_matching.py

[rossburton]

In case a human reviewer doesn't notice, there's also #1407.