setup: build extensions in parallel by default The build_ext distutils command in Python 3.5+ has a "parallel" option that controls whether to build extensions in parallel. It is disabled by default (None) and can be set to an integer value for number of cores or True to indicate use all available CPU cores. This commit changes our build_ext command override to set "parallel" to True unless a value has been provided by the caller. On my machine, this makes `python setup.py build_ext` 1-4s faster. It is worth noting that at this time, each individual source file constituting the extension is still built serially. For Mercurial, this means that we can't build faster than the slowest-to-build extension, which is the zstd extension by a long shot. This means that setup.py is still not very efficient at utilizing multiple cores. But we're better than before. Differential Revision: https://phab.mercurial-scm.org/D6923 # no-check-commit because of foo_bar naming

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