contrib: update to latest Python 2.7, 3.7, and 3.8
We would ideally update the 3.5 and 3.6 versions as well. But Python
didn't generate exe installers for newer versions for some reason.
Differential Revision: https://phab.mercurial-scm.org/D8466
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true