singlehead: making config item a bool again with the use of `configsuboptions`, the main config item has become a string (unless it's just the default value). This makes it in particular hard to override in a cascade of HGRC files, as we do in Heptapod to re-allow multiple heads on specific repositories while the default behaviour is to forbid them. The added test case reflects that use-case

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