Parse long strings with \U codes.

# Bug report

### Bug description:


```python
# The following parses just fine:
line1="""
this has \u21A9 in it
"""
# This fails to parse:
line2="""
this has \U21A9 in it
"""
```

The error message is `unicodeescape' codec can't decode bytes in position...`



### CPython versions tested on:

3.13

### Operating systems tested on:

macOS