Fix type annotation for import_model_module (AMICI-dev#2282)

The second argument never was optional.
Not sure whether there would be any good default value.

python/sdist/amici/__init__.py

@@ -13,7 +13,7 @@
 import sys
 from pathlib import Path
 from types import ModuleType as ModelModule
-from typing import Any, Callable, Optional, Union
+from typing import Any, Callable, Union
 
 
 def _get_amici_path():
@@ -153,7 +153,7 @@ def __exit__(self, exc_type, exc_value, traceback):
 
 
 def import_model_module(
-    module_name: str, module_path: Optional[Union[Path, str]] = None
+    module_name: str, module_path: Union[Path, str]
 ) -> ModelModule:
     """
     Import Python module of an AMICI model