[MNT] address deprecation of load_module (#190)

`load_module` is deprecated and will be replaced by `exec_module` in
python 3.12, this PR addresses that.

The fix is highly nontrivial - the crucial line was
`imported_mod = module.load_module()` for a `module: SourceFileLoader`,

which had to be replaced by the equivalent four (!) lines

```python
spec = importlib.util.spec_from_file_location(module.name, module.path)
imported_mod = importlib.util.module_from_spec(spec)

# these two are also crucial due to the side effects
loader = spec.loader
loader.exec_module(imported_mod)
```

The deprecation message raised by `load_module` is highly unhelpful, and I had to reverse engineer this from a very helpful PR in `nox`, wntrblm/nox#498

skbase/lookup/_lookup.py

@@ -297,7 +297,11 @@ def _import_module(
         if isinstance(module, str):
             imported_mod = importlib.import_module(module)
         elif isinstance(module, importlib.machinery.SourceFileLoader):
-            imported_mod = module.load_module()
+            spec = importlib.util.spec_from_file_location(module.name, module.path)
+            imported_mod = importlib.util.module_from_spec(spec)
+
+            loader = spec.loader
+            loader.exec_module(imported_mod)
         exc = None
     except Exception as e:
         # we store the exception so we can restore the stdout fisrt