Split python-tornado packages by version. tornado 5 is the version supported by the most packages, and the default. Some packages still only work with tornado 4. tornado 6 is the most recent version. Having split versions allows packages that depend on a particular version to still work.

This is the python-tornado version 4 package. This is will stop working with python 3.8, so it automatically disables python 3 builds for that version.