Implement Py_mod_create slot support in multi-phase init

Following CPython's implementation, now properly use the create slot
if provided, instead of always using default module creation.

CPython flow (Objects/moduleobject.c PyModule_FromDefAndSpec):
- If Py_mod_create slot exists: call create(spec, def)
- Otherwise: use PyModule_NewObject() for default creation

RustPython now follows the same pattern:
- If slots.create exists: call create(vm, spec, def)
- Otherwise: use PyModule::from_def() + __init_dict_from_def()

This allows modules to provide custom module objects during creation
phase, enabling advanced use cases like singleton modules or modules
with special initialization requirements.

Tested:
- Basic imports work correctly
- sha256/sha512 with custom exec still work
- test_pickle, test_pickletools, test_hashlib all pass

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: youknowone

crates/vm/src/import.rs

@@ -101,11 +101,17 @@ pub fn import_builtin(vm: &VirtualMachine, module_name: &str) -> PyResult {
     if let Some(def_func) = vm.state.module_defs.get(module_name) {
         let def = def_func(&vm.ctx);
 
-        // Phase 1: Create module from definition
-        let module = PyModule::from_def(def).into_ref(&vm.ctx);
-
-        // Initialize module dict using proper method
-        PyModule::__init_dict_from_def(vm, &module);
+        // Phase 1: Create module (use create slot if provided, else default creation)
+        let module = if let Some(create) = def.slots.create {
+            // Custom module creation (CPython: Py_mod_create slot)
+            let spec = vm.ctx.new_str(module_name);
+            create(vm, spec.as_object(), def)?
+        } else {
+            // Default module creation
+            let module = PyModule::from_def(def).into_ref(&vm.ctx);
+            PyModule::__init_dict_from_def(vm, &module);
+            module
+        };
 
         // Add to sys.modules BEFORE exec (critical for circular import handling)
         sys_modules.set_item(module_name, module.clone().into(), vm)?;