Py3: symbolic/expression.pyx fix some errors

[vklein]

• Fix the errors AttributeError: 'sage.symbolic.expression.Expression' object has no attribute '__nonzero__' appearing in python3.

'__nonzero__' is renamed into '__bool__' in python3.

• Use #py2 #py3 tag to manage the difference of messages for TypeError when an object is not subscriptable.

• Fix the following doctest:

  sage: Set([-x + y < 0, x - y < 0])
  {-x + y < 0, x - y < 0}
  

The later used to randomly fail because the output order was not constant with python3.

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​7cf1dab

Trac #27885: expression.pyx fix __nonzero__ ...

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​2a2f195

Trac #27885: expression.pyx fix __nonzero__ ...

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​7658b8e

Trac #27885: expression.pyx ...

[chapoton]

ok, thx