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Bump setuptools_scm and other skeleton updates, including:
- Drop support for `setup.py build_sphinx` because that usage
conflicts with the usage required by readthedocs, which
always runs the docs build from `./docs/` instead of `./`.
Also drop support for build_sphinx because the dependency
resolution of that command relies on easy_install, which is
also deprecated. The proper way to build docs now is to
install the requirements in docs/requirements.txt and then
invoke sphinx natively.