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_dummy_clear.py

@@ -0,0 +1,8 @@
+from escpos.printer import Dummy
+
+
+def test_printer_dummy_clear() -> None:
+    printer = Dummy()
+    printer.text("Hello")
+    printer.clear()
+    assert printer.output == b""