Constants can't be deepcopied

```python
In [1]: copy.deepcopy(astropy.constants.c)
ERROR:astropy:TypeError: __new__() takes at least 7 arguments (2 given)
ERROR: TypeError: __new__() takes at least 7 arguments (2 given) [astropy.units.quantity]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-26-15520179d3fa> in <module>()
----> 1 copy.deepcopy(astropy.constants.c)

/usr/lib/python2.7/copy.pyc in deepcopy(x, memo, _nil)
    172             copier = getattr(x, "__deepcopy__", None)
    173             if copier:
--> 174                 y = copier(memo)
    175             else:
    176                 reductor = dispatch_table.get(cls)

/usr/lib/python2.7/site-packages/astropy/constants/constant.pyc in wrapper(self, *args, **kwargs)
     56                                                       systems[0]))
     57 
---> 58                 return meth(self, *args, **kwargs)
     59 
     60             return wrapper

/usr/lib/python2.7/site-packages/astropy/units/quantity.pyc in __deepcopy__(self, memo)
    409         # If we don't define this, ``copy.deepcopy(quantity)`` will
    410         # return a bare Numpy array.
--> 411         return self.copy()
    412 
    413     def __quantity_view__(self, obj, unit):

/usr/lib/python2.7/site-packages/astropy/units/quantity.pyc in copy(self)
    554     def copy(self):
    555         """ Return a copy of this `Quantity` instance """
--> 556         return self.__class__(self)
    557 
    558     # This flag controls whether convenience conversion members, such

TypeError: __new__() takes at least 7 arguments (2 given)
```

Thanks to @transientlunatic for spotting this.