Add weakref support for Symbols, fix sympy/sympy#8825

At this commit::

    $ ./bin/isympy -q
    Python console for SymPy 0.7.6-git (Python 2.7.3-64-bit) (ground types: python)

    >>> import weakref
    >>> d = weakref.WeakKeyDictionary([(x,1), (y, 2)])
    >>> print(d.items())
    [(x, 1), (y, 2)]
    >>> del x
    >>> print(d.items())
    [(y, 2)]

sympy/core/symbol.py

@@ -34,7 +34,7 @@ class Symbol(AtomicExpr, Boolean):
 
     is_comparable = False
 
-    __slots__ = ['name']
+    __slots__ = ['name', '__weakref__']
 
     is_Symbol = True