revlog: move the "index header" struct inside revlog.utils.constants
The struct was previous called "version", but this is actually "version" +
"flags". So header seems like a better name.
The move to the `constants` module has the same motivation as the INDEX_ENTRY_V#
ones.
Differential Revision: https://phab.mercurial-scm.org/D10306
[build-system]
requires = ["setuptools", "wheel"]
build-backend = "setuptools.build_meta"
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true