More mypy (#612)

* remove type comment where type is annotated
* move function tests
* remove six from tests
* add none annotations
* add more types
* change mock (so that mypy understands it)

test/test_functions/test_function_cashdraw.py

@@ -0,0 +1,14 @@
+#!/usr/bin/python
+
+import pytest
+
+import escpos.printer as printer
+from escpos.exceptions import CashDrawerError
+
+
+def test_raise_CashDrawerError() -> None:
+    """should raise an error if the sequence is invalid."""
+    instance = printer.Dummy()
+    with pytest.raises(CashDrawerError):
+        # call with sequence that is too long
+        instance.cashdraw([1, 1, 1, 1, 1, 1])