Explicitly mark arguments as optional

mypy now considers implicitly Optional arguments
to be errors - I have made them now explicitly
Optional, which passes.

openflexure_microscope/camera/pi.py

@@ -30,7 +30,7 @@ import logging
 import time
 
 # Type hinting
-from typing import BinaryIO, Tuple, Union
+from typing import BinaryIO, Tuple, Union, Optional
 
 import numpy as np
 
@@ -307,7 +307,7 @@ class PiCameraStreamer(BaseCamera):
             self.picamera.zoom = new_fov
 
     def start_preview(
-        self, fullscreen: bool = True, window: Tuple[int, int, int, int] = None
+        self, fullscreen: bool = True, window: Optional[Tuple[int, int, int, int]] = None
     ):
         """Start the on board GPU camera preview."""
         with self.lock(timeout=1):
@@ -462,9 +462,9 @@ class PiCameraStreamer(BaseCamera):
         output: Union[str, BinaryIO],
         fmt: str = "jpeg",
         use_video_port: bool = False,
-        resize: Tuple[int, int] = None,
+        resize: Optional[Tuple[int, int]] = None,
         bayer: bool = True,
-        thumbnail: Tuple[int, int, int] = None,
+        thumbnail: Optional[Tuple[int, int, int]] = None,
     ):
         """
         Capture a still image to a StreamObject.

openflexure_microscope/config.py

@@ -3,6 +3,8 @@ import json
 import logging
 import os
 import shutil
+from typing import Optional
+
 
 from .json import JSONEncoder
 from .paths import CONFIGURATION_FILE_PATH, SETTINGS_FILE_PATH
@@ -17,7 +19,7 @@ class OpenflexureSettingsFile:
         expand (bool): Expand paths to valid auxillary config files.
     """
 
-    def __init__(self, path: str, defaults: dict = None):
+    def __init__(self, path: str, defaults: Optional[dict] = None):
         defaults = defaults or {}
 
         # Set arguments

openflexure_microscope/microscope.py

@@ -370,7 +370,7 @@ class Microscope:
         folder: str = "",
         temporary: bool = False,
         use_video_port: bool = False,
-        resize: Tuple[int, int] = None,
+        resize: Optional[Tuple[int, int]] = None,
         bayer: bool = True,
         fmt: str = "jpeg",
         annotations: Optional[Dict[str, str]] = None,