Use ClauseElement.__str__() instead of str() to obtain statement

When an field value is a unicode string, the use of str() in python 2.x
will cause the str() call to fail. Python 3.x does not have a unicode()
function, however SQLAlchemy does have __str__() which will do
encoding between py2 and py3.

https://github.com/sqlalchemy/sqlalchemy/blob/master/lib/sqlalchemy/sql/elements.py#L489

flask_sqlalchemy_caching/core.py

@@ -129,7 +129,7 @@ def key_from_query(self, qualifier=None):
         particular Query, then combining that with the bound parameter values.
         """
         stmt = self.with_labels().statement
-        key = str(stmt.compile(compile_kwargs={'literal_binds': True}))
+        key = stmt.compile(compile_kwargs={'literal_binds': True}).__str__()
         return md5(key.encode('utf8')).hexdigest()