gh-154387: fix structseq_repr by labeling unnamed fields in repr as <unnamed@N>

[XuehaiPan]

Walk the members alongside the visible slots and label unnamed slots positionally as <unnamed@N>, which is correct regardless of where the unnamed fields sit among the visible ones.

Before this PR, the member slots are accessed incorrectly in repr when there are unnamed fields. It can cause a spurious SystemError when the index reaches the number of named fields (n_fields - n_unnamed_fields), or display the unnamed fields with <name>=<value> using a later field's name.

For example, a structseq type with WithUnnamed(first, <unnamed>, third, <unnamed>) + [hidden .fifth], before this PR:

>>> seq = WithUnnamed(range(5))
>>> repr(seq)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    repr(cls(range(5)))
    ~~~~^^^^^^^^^^^^^^^
SystemError: In structseq_repr(), member 3 name is NULL for type WithUnnamed
# -> WithUnnamed(first=0, third=1, fifth=2, <reach NULL-terminated tp_members' end for member 3>)

After this PR:

>>> seq = WithUnnamed(range(5))
>>> seq
WithUnnamed(first=0, <unnamed@1>=1, third=2, <unnamed@3>=3)
>>> seq.fifth
4

---

Before this PR, for os.stat_result, the unnamed slots printed as the following field names st_atime/st_mtime/st_ctime.

>>> st = os.stat_result(range(os.stat_result.n_fields))
>>> st
os.stat_result(st_mode=0, st_ino=1, st_dev=2, st_nlink=3, st_uid=4, st_gid=5, st_size=6,
               st_atime=7, st_mtime=8, st_ctime=9)  # borrow the following fields' names but use the unnamed fields' values
>>> st[7]
7
>>> st.st_atime
10

>>> st = os.stat_result(tuple(range(10)) + (7.1, 8.2, 9.3))
>>> st
os.stat_result(st_mode=0, st_ino=1, st_dev=2, st_nlink=3, st_uid=4, st_gid=5, st_size=6,
               st_atime=7, st_mtime=8, st_ctime=9)
>>> st[7]        # index = 7 is int
7
>>> st.st_atime  # index = 10 is float
7.1

After this PR, a new custom statresult_repr is added analogously to statresult_new. The custom repr shows the fields 10-12 (always initialized in statresult_new) instead of generic <unnamed@N>.

>>> st = os.stat_result(range(os.stat_result.n_fields))
>>> st
os.stat_result(st_mode=0, st_ino=1, st_dev=2, st_nlink=3, st_uid=4, st_gid=5, st_size=6,
               st_atime=10, st_mtime=11, st_ctime=12)  # make the field names and values match
>>> st[7]
7
>>> st.st_atime
10

>>> st = os.stat_result(tuple(range(10)) + (7.1, 8.2, 9.3))
>>> st
os.stat_result(st_mode=0, st_ino=1, st_dev=2, st_nlink=3, st_uid=4, st_gid=5, st_size=6,
               st_atime=7.1, st_mtime=8.2, st_ctime=9.3)
>>> st[7]        # index = 7 is int
7
>>> st.st_atime  # index = 10 is float
7.1

>>> os.stat(os.curdir)
os.stat_result(st_mode=16877, st_ino=429294614, st_dev=16777232, st_nlink=58, st_uid=501, st_gid=20, st_size=1856,
               st_atime=1784996316.4868717, st_mtime=1784996316.472091, st_ctime=1784996316.472091)

Without the custom statresult_repr, the generic <unnamed@N> will be used, which is not that legible.

>>> st = os.stat_result(range(os.stat_result.n_fields))
>>> st
os.stat_result(st_mode=0, st_ino=1, st_dev=2, st_nlink=3, st_uid=4, st_gid=5, st_size=6,
               <unnamed@7>=7, <unnamed@8>=8, <unnamed@9>=9)

>>> os.stat(os.curdir)
os.stat_result(st_mode=16877, st_ino=429294614, st_dev=16777232, st_nlink=58, st_uid=501, st_gid=20, st_size=1856,
               <unnamed@7>=1784996316, <unnamed@8>=1784996316, <unnamed@9>=1784996316)

Fixes #154387

• Issue: structseq_repr mislabels unnamed PyStructSequence fields (e.g. os.stat_result slots 7–9) #154387