chg:pkg: Upgrade current build and test systems

Changes:

• move bin/duplicity to duplicity/main.py
• add entry point dup_run() no args
• rename bin to man (only contents now)
• rename duplicity/tarfile.py to duplicity/dup_tarfile.py to avoid import problems
• duplicity now runs as a module python3 -m duplicity as well as a script /usr/bin/duplicity
• py2->py3 oddities changed, "".__class__ and b"".__class__ changed to str and bytes
• tox v4 now runs correctly as tox run -e code
• moved [pycodestyle] from tox.ini to setup.cfg
• moved .pylintrc from to setup.cfg
• sources released fully versioned
  • duplicity/__init__.py
  • man/duplicity.1
  • pyproject.toml
  • setup.py
  • snap/snapcraft.yaml

Closes #774 (closed),#793 (closed)