Make Nothing class derive from object

Otherwise it behaves like an old-style class of type classobj.  All instances
of these old classe are of type 'instance' so there is no way to do clean type
checking.

  modified:   pytriqs/base/gf/local/nothing.py
Make python gf tests more portable

Otherwise there were relative comparisons of 1e-16 and 0...

  modified:   pytriqs/base/test/CMakeLists.txt