Parse long strings with \U codes.

[mjbmon]

Bug report

Bug description:

# 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

[mjbmon]

If you use \U constant in string then you need to specify 8 bytes, not four.

line="\U00002148"
# or
line="""
\U00002148
"""