Ensure XML files are opened with UTF-8 encoding

foyer/forcefield.py

@@ -66,7 +66,7 @@ def preprocess_forcefield_files(forcefield_files=None):
 
     for xml_file in forcefield_files:
         if not hasattr(xml_file, "read"):
-            f = open(xml_file)
+            f = open(xml_file, encoding="utf-8")
             _, suffix = os.path.split(xml_file)
         else:
             f = xml_file