Fix importlib import

On Python versions < 3.10 (I think I'm correct in saying this fails for 3.9), running `pytest` fails if using `importlib.resources` (see mu-editor/pup#35). This can be fixed by using `importlib_resources` instead (GitHub Actions testing should work now for all tested versions of Python).

atomdb/periodic.py

@@ -1,7 +1,7 @@
 import csv
 import numpy as np
 
-from importlib.resources import files
+from importlib_resources import files
 
 __all__ = ["sym2num", "name2num", "num2sym", "num2name", "Atom"]