strptime: use %r in error messages, not %s

Bug report

Bug description:

Consider the following example, say I'm parsing a date string read from an external source, and I forgot to call .strip():

from datetime import datetime

datetime.strptime("2001-02-03\n", "%Y-%m-%d")

The output is a little confusing:

$ python3 strptime_example.py
Traceback (most recent call last):
  File "strptime_example.py", line 3, in <module>
    datetime.strptime("2025-11-14\n", "%Y-%m-%d")
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/_strptime.py", line 789, in _strptime_datetime
    tt, fraction, gmtoff_fraction = _strptime(data_string, format)
                                    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/_strptime.py", line 558, in _strptime
    raise ValueError("unconverted data remains: %s" %
                      data_string[found.end():])
ValueError: unconverted data remains: 

The error message is using %s, when using %r would give a more unambiguous description of the extra data – similar with strings quoted manually using ' in the error message just above:

https://github.com/python/cpython/blob/a486d452c78a7dfcd42561f6c151bf1fef0a756e/Lib/_strptime.py#L567-L570

CPython versions tested on:

3.13

Operating systems tested on:

macOS

Linked PRs

• gh-141541
• gh-141712

Empieza en Lib/_strptime.py alrededor del error de datos no convertidos en las líneas citadas en el informe y reproduce el ejemplo con el salto de línea final. El trabajo estará terminado cuando los datos restantes se muestren sin ambigüedad usando un formato de estilo repr, coherente con el mensaje de error cercano.