The install routine was written before the fix in #8241, and wasn't
sufficiently tested with that fix.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

This renames the Python 2 twisted package to python-twisted, and updates
dependents (i.e. obfsproxy) to reference the updated name.

This also fixes conflicts between the Python 2 and 3 packages. Twisted
installs some scripts to /usr/bin, and previously scripts for both
packages used the same names. This adds a "3" suffix to scripts
installed by python3-twisted.

This also adds python[3]-setuptools as a dependency, as the scripts
installed to /usr/bin depend on pkg_resources (part of setuptools).

Signed-off-by: Jeffery To <jeffery.to@gmail.com>