symlinks /usr/bin/python to python3

 Starting with the Debian 11 (bullseye) and Ubuntu 20.04 LTS (focal)
 releases, all python packages use explicit python3 or python2
 interpreter and do not use unversioned /usr/bin/python at all. Some
 third-party code is now predominantly python3 based, yet may use
 /usr/bin/python.
 .
 This is a convenience package which ships a symlink to point
 the /usr/bin/python interpreter at the current default python3. It may
 improve compatibility with other modern systems, whilst breaking some
 obsolete or 3rd-party software.
 .
 No packages may declare dependencies on this package.