Install fastnumbers for mypy

Otherwise it complains that it cannot find the types, understandably.

.github/workflows/code-quality.yml

@@ -59,7 +59,7 @@ jobs:
           python-version: '3.6'
 
       - name: Install MyPy
-        run: pip install mypy hypothesis pytest pytest-mock
+        run: pip install mypy hypothesis pytest pytest-mock fastnumbers
 
       - name: Run MyPy
         run: mypy --strict natsort tests

tox.ini

@@ -59,6 +59,7 @@ deps =
     hypothesis
     pytest
     pytest-mock
+    fastnumbers
 commands =
     mypy --strict natsort tests
 skip_install = true