Fix wmlparser3 considers "_" part of attribute value

The wml parser, when parsing the outside of a string that is
translatable, only assumed “spaces” characters could precede it. If
there are “tabs” characters the code parses the “_” character as an
attribute value.

The solution is to add a stripping code for “tabs” characters.

Code style updated based on recommendations from ProditorMagnus.

data/tools/wesnoth/wmlparser3.py

@@ -519,7 +519,7 @@ def parse_outside_strings(self, line):
                 self.handle_attribute(line)
         else:
             for i, segment in enumerate(line.split(b"+")):
-                segment = segment.lstrip(b" ")
+                segment = segment.lstrip(b" \t")
 
                 if i > 0:
                     # If the last segment is empty (there was a plus sign