convert: use bytes for value in extra dict

The keys and values of the extra dict are expected to be bytes. Before this
fix, there was a crash in mercurial.changelog.encodeextra().
10 jobs for branch/stable in 34 minutes and 57 seconds (queued for 2 seconds)
Name Stage Failure
failed
test-py3-rust Test
Skipped test-sparse-fsmonitor.t: skipped
Skipped test-sparse-revlog.t: missing artifact, run "/tmp/mercurial-ci/tests/artifacts/scripts/generate-churning-bundle.py"
Skipped test-sqlitestore.t: missing feature: sqlite3 module is available
Skipped test-verify-repo-operations.py: missing feature: allow slow tests (use --allow-slow-tests)
Skipped test-wireproto-exchangev2-shallow.t: missing feature: sqlite3 module is available
Failed test-strip.t: output changed
# Ran 861 tests, 60 skipped, 1 failed.
python hash seed: 874259075
ERROR: Job failed: exit code 1