[mypy] Adding mypy linting (#7053)

CONTRIBUTING.md

@@ -462,6 +462,35 @@ Note that the test environment uses a temporary directory for defining the
 SQLite databases which will be cleared each time before the group of test
 commands are invoked.
 
+#### Typing
+
+To ensure clarity, consistency, all readability, _all_ new functions should use
+[type hints](https://docs.python.org/3/library/typing.html) and include a
+docstring using Sphinx documentation.
+
+Note per [PEP-484](https://www.python.org/dev/peps/pep-0484/#exceptions) no
+syntax for listing explicitly raised exceptions is proposed and thus the
+recommendation is to put this information in a docstring, i.e.,
+
+
+```python
+import math
+from typing import Union
+
+
+def sqrt(x: Union[float, int]) -> Union[float, int]:
+    """
+    Return the square root of x.
+
+    :param x: A number
+    :returns: The square root of the given number
+    :raises ValueError: If the number is negative
+    """
+
+    return math.sqrt(x)
+```
+
+
 ### JavaScript Testing
 
 We use [Jest](https://jestjs.io/) and [Enzyme](https://airbnb.io/enzyme/) to test Javascript. Tests can be run with: