Cleanup: fix mypy errors

This mostly handles a mypy change where implicit optionals (i.e. things
like `size: int = None`) are no longer allowed.

No functional changes.

blender_asset_tracer/pack/progress.py

@@ -25,6 +25,7 @@ import logging
 import pathlib
 import queue
 import typing
+from typing import Optional
 
 import blender_asset_tracer.trace.progress
 
@@ -134,7 +135,7 @@ class ThreadSafeCallback(Callback):
     def missing_file(self, filename: pathlib.Path) -> None:
         self._queue(self.wrapped.missing_file, filename)
 
-    def flush(self, timeout: float = None) -> None:
+    def flush(self, timeout: Optional[float] = None) -> None:
         """Call the queued calls, call this in the main thread."""
 
         while True:

blender_asset_tracer/pack/transfer.py

@@ -25,6 +25,7 @@ import queue
 import threading
 import time
 import typing
+from typing import Optional
 
 from . import progress
 
@@ -186,7 +187,7 @@ class FileTransferer(threading.Thread, metaclass=abc.ABCMeta):
                 if self.done.is_set():
                     return
 
-    def join(self, timeout: float = None) -> None:
+    def join(self, timeout: Optional[float] = None) -> None:
         """Wait for the transfer to finish/stop."""
 
         if timeout: