Removal of py2 code

[sveinse]

• Remove packages future and attrs
• Fixup spacing on all copyright texts (black will change that)
• Ordered all imports (with isort)
• Remove object usage
• Replace call unicode instances with str, long with int
• Replace raise_from with raise ... from
• Remove all usages of OrderedDict. Ordinary dict guarantees order
• Replace execfile with exec
• Combine nosis/pickler.py and nosis/xtoy.py into nosis.py
• Added tests for nosis.py
• Raise NotImplementedError on node BE_to_LE() and LE_to_BE() as they might not work properly in py3. Missing test suits for them to verify.