Bump iohub to 0.3.2, requires-python to >=3.12

iohub 0.3.1+ replaced multiprocessing.Pool with ThreadPoolExecutor
in process_single_position (czbiohub-sf/iohub#396), renaming
num_processes to num_threads. The old num_threads kwarg was leaking
through **kwargs to transform functions, causing TypeError in
run-flat-field.

iohub 0.3.2 also restores OME-Zarr v0.4 write support. Both 0.3.1
and 0.3.2 require Python >=3.12.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: aofei-liu

pyproject.toml

@@ -7,7 +7,7 @@ name = "biahub"
 description = "Bioimage analysis hub."
 readme = "README.md"
 license = { file = "LICENSE" }
-requires-python = ">=3.11"
+requires-python = ">=3.12"
 
 dynamic = ["version"]
 
@@ -20,9 +20,7 @@ classifiers = [
 
 dependencies = [
   "click",
-  # need to pin until #198, #199, #215 are resolved
-  # iohub>0.3 writes to zarr v3 by default which breaks biahub tests
-  "iohub==0.3.0a7",
+  "iohub==0.3.2",
   "stitch @ git+https://github.com/ahillsley/stitching@jen",
   "matplotlib",
   "natsort",