Drop Python 2 support

[JuanTecedor]

• Encoding special comment line is still required if there is a shebang as per https://docs.python.org/3/tutorial/interpreter.html#source-code-encoding.
• Changed special comment line (encoding) to # -*- coding: utf-8 -*- form
• Super does not require class name https://peps.python.org/pep-3135/
• Modernize % format strings to f-strings
• Drop from builtins import str as unicode (all text should be unicode https://docs.python.org/3/whatsnew/3.0.html#text-vs-data-instead-of-unicode-vs-8-bit)
• Use with statement instead of raw open
• Drop all sys.version_info checks and branches
• Remove inherit from object (actually this is a prerequisite if we want to make the Worker ABC later)

https://docs.python.org/3/whatsnew/3.0.html