Update generate_proto_list.py (cyberbotics#6523)

In Chinese Windows System，”open“ will  generate error: 

self.contents = file.read()
                    ^^^^^^^^^^^
UnicodeDecodeError: 'gbk' codec can't decode byte 0x93 in position 547: illegal multibyte sequence

Author: FrogGuaGuaGua

scripts/packaging/generate_proto_list.py

@@ -44,7 +44,7 @@ def __init__(self, path, name):
         self.needs_robot_ancestor = False
 
         # store file contents to avoid reading it multiple times
-        with open(self.path, 'r') as file:
+        with open(self.path, 'r', encoding='utf-8') as file:
             self.contents = file.read()
             # remove IndexedFaceSet related fields since they significantly slow down the subsequent regex
             self.contents = re.sub(r'point\s+\[[^\]]+\]', '', self.contents)