py3: fix formatting of LookupError for workingctx
Spotted while writing broken tests for "hg grep -fr'wdir()'".
basectx._fileinfo() raises ManifestLookupError(self._node, ..), but _node
of the workingctx is None for historical reasons.
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true