Add static type hints checking (qiskit-community/qiskit-aqua#1020)

* Add static type hints checking

* run mypy on python 3.5

* a few more type hint fixes

Co-authored-by: Julien Gacon <jules.gacon@googlemail.com>

qiskit/aqua/components/variational_forms/variational_form.py

@@ -43,7 +43,7 @@ def __init__(self) -> None:
         super().__init__()
         self._num_parameters = 0
         self._num_qubits = 0
-        self._bounds = list()
+        self._bounds = list()  # type: List[object]
         self._support_parameterized_circuit = False
         pass
 
@@ -95,7 +95,7 @@ def num_qubits(self):
         return self._num_qubits
 
     @property
-    def parameter_bounds(self):
+    def parameter_bounds(self) -> List[object]:
         """Parameter bounds.
 
         Returns: