fix list parsing edge case

fix an incorrect list parsing (text dropped) in situations like:

```
1. item 1
2. \xA0 item 2
```

The line text is dropped because first non-space sharacter happens
to match `\s` regex pattern.

There are alternative fixes:

* change regex to `( *)[^ ]`.
* use `text.isspace()` and `text.lstrip()` instead of regex matching.

src/mistune/list_parser.py

@@ -13,7 +13,7 @@
     r'(?P<list_3>[ \t]*|[ \t].+)$'
 )
 
-_LINE_HAS_TEXT = re.compile(r'( *)\S')
+_LINE_HAS_TEXT = re.compile(r'(\s*)\S')
 
 
 def parse_list(block, m: re.Match, state: BlockState) -> int: