Exclude generated files in docs/_build from the Python sdist

[pronovic]

Due to limitations in Poetry's [tool.poetry] configuration block, there's no simple way to include docs/ in the sdist, while also excluding the generated files in docs/_build. The cleanest workaround is to remove docs/_build before running the poetry build command. That's clearly a hack, but there's apparently no other alternative. Technically, this is only necessary for projects that build docs, but it's easiest to just always do the cleanup.