dispatch: move IOError handling and flushing of streams to `dispatch()`
Instead of patching both dispatch code and commandserver code, we directly
handle this in `dispatch.dispatch()`.
Thanks to Yuya who recommended this.
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true