get_plugin_data: always use utf-8 encoding to read plugins

Fixes build issues on systems using a different default encoding (e.g. Windows)

get_plugin_data.py

@@ -17,7 +17,7 @@
 def get_plugin_data(filepath):
     """Parse a python file and return a dict with plugin metadata"""
     data = {}
-    with open(filepath, 'rU') as plugin_file:
+    with open(filepath, 'rU', encoding='utf-8') as plugin_file:
         source = plugin_file.read()
         try:
             root = ast.parse(source, filepath)