Minor: Fix a Cython declaration in a Reaction method.

Although Atom is a subclass of Vertex, it adds an element
attribute. This leads to more efficient Cython code
(and quietens a warning in vscode).

rmgpy/reaction.py

@@ -1189,7 +1189,7 @@ def is_balanced(self):
  from rmgpy.molecule.element import element_list
  from rmgpy.molecule.fragment import CuttingLabel, Fragment
 
- cython.declare(reactant_elements=dict, product_elements=dict, molecule=Graph, atom=Vertex, element=Element)
+ cython.declare(reactant_elements=dict, product_elements=dict, molecule=Molecule, atom=Atom, element=Element)
 
  reactant_elements = {}
  product_elements = {}