v3.4.0

Release 3.4.0 (#11)

#10 shipped the sealed Cardmarket listing methods and the opened and
sell_count fields but left the version at 3.3.0, so PyPI still serves a
3.3.0 that does not have them. Nothing is published until the version
moves.

Minor rather than patch: sealed gained cardmarket/iterate_cardmarket/
all_cardmarket on both the sync and async clients, and CardmarketListing
gained two fields. Every addition, nothing renamed or removed, so a
3.3.0 caller keeps working.

Both files, because the version lives twice: pyproject.toml is what the
build reads and __init__.py is what __version__ reports, and a release
that moves only one makes the installed package disagree with itself.