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/blendfile/dna.py

@@ -22,6 +22,7 @@
 import logging
 import os
 import typing
+from typing import Optional
 
 from . import header, exceptions
 
@@ -104,7 +105,7 @@ class Struct:
 
     log = log.getChild("Struct")
 
-    def __init__(self, dna_type_id: bytes, size: int = None) -> None:
+    def __init__(self, dna_type_id: bytes, size: Optional[int] = None) -> None:
         """
         :param dna_type_id: name of the struct in C, like b'AlembicObjectPath'.
         :param size: only for unit tests; typically set after construction by

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:

blender_asset_tracer/trace/result.py

@@ -21,6 +21,7 @@ import functools
 import logging
 import pathlib
 import typing
+from typing import Optional
 
 from blender_asset_tracer import blendfile, bpathlib
 from blender_asset_tracer.blendfile import dna
@@ -57,9 +58,9 @@ class BlockUsage:
         block: blendfile.BlendFileBlock,
         asset_path: bpathlib.BlendPath,
         is_sequence: bool = False,
-        path_full_field: dna.Field = None,
-        path_dir_field: dna.Field = None,
-        path_base_field: dna.Field = None,
+        path_full_field: Optional[dna.Field] = None,
+        path_dir_field: Optional[dna.Field] = None,
+        path_base_field: Optional[dna.Field] = None,
         block_name: bytes = b"",
     ) -> None:
         if block_name: