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Version 3.0 reflects that we dropped support for Python 2.
All the other metadata were updated as well.

(Version 2.y could be the new parser but I forgot to update
the setup.py back then.)

Right now I'm not aware of any reason to invest into Python 2 any further.

Deckard now can run multiple processes inside single simulated network
and communicate with each other.

This introduces couple of incompatible changes:
- command line syntax was changed to accomodate new possibilities (see --help)
- SOCKET_WRAPPER environment variables except DIR are always managed by Deckard

Cheap re-indentation using python-autopep8-1.2.1-3.fc25 with few manual
tweaks for very long lines.

This costs nothing and will avoid PEP8 complaints about whitespace in CI.

Linux distributions are moving towards Python 3. As a consequence,
staying on Python 2 is causing problems. E.g. Deckard errored out
on missing 'dns' package but 'pip install' claimed that 'dns' is already
installed...

From now the Deckard is using default Python version so it should be
less surprising. If necessary, the default version can be overriden
using PYTHON variable in Makefile.

An important side-effect is that Deckard running under Python 3 is
easier to debug because Python 3 is able to report whole chains
of exceptions properly.

This fixes installations where python3 is the default.