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().
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true