Fix yaml.emit for percent-escaped tag prefixes

[ShipItAndPray]

Fixes #862.

Summary

• stop calling ord() on bytes iteration results in Emitter.prepare_tag_prefix()
• add a regression test for emitting a %TAG directive parsed from bytes with a percent-escaped prefix

Repro

PYTHONPATH=lib python3 - <<'PY'
import io
import yaml

text = io.BytesIO(b"%TAG ! tag:%002:\n---")
events = list(yaml.parse(text))
print(yaml.emit(events))
PY

Validation

• PYTHONPATH=lib python3 -m pytest tests/test_emit.py -q
• PYTHONPATH=lib python3 -m pytest tests/test_dump_load.py tests/test_emit.py -q
• PYTHONPATH=lib PYYAML_FORCE_LIBYAML=0 python3 -m pytest tests/legacy_tests/test_yaml.py -q -k emitter