Better approach to `is_mapped_class`

`sqlalchemy.orm.class_mapper` relies on `sqlalchemy.orm.configure_mappers` to have been called - either explicitly or implicitly by attempting to interact with the mapper. This means that `graphene_sqlalchemy.utils.class_mapper` forces a call to `configure_mappers` – which in turn throws an error unless every model referenced in a deferred callable (i.e. `user = relationship('User')`) has already been discovered.

However, there is another way to determine if a class is a mapper without forcing `configure_mappers`. The recipe is this:

```python
from sqlalchemy.orm import Mapper
from sqlalchemy.exc import NoInspectionAvailable

def is_mapped_class(cls):
'''modified from graphene_sqlalchemy/types.py '''
try:
return isinstance(inspect(cls), Mapper)
except NoInspectionAvailable:
return False
```