tests: write out file using bytes I/O
The encoding of sys.stdout varies between Python versions. So
using a one-liner to write a file from a Unicode string is not
deterministic.
This commit writes out the file using bytes I/O to ensure we
have exactly the bytes we want in the file.
This change fixes a test failure in Python 3.5/3.6.
Differential Revision: https://phab.mercurial-scm.org/D7226
[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