Support GS1-128 date AIs with a four-digit year (7250, 7251)

[gaoflow]

GS1 Application Identifiers 7250 (DOB) and 7251 (DOB TIME) carry a four-digit
year (N8 = YYYYMMDD, N12 = YYYYMMDDhhmm), but gs1_128 only handled
two-digit-year date formats, so both AIs were broken in either direction:

• info('725019800715') raised a bare ValueError: time data '19800715' does not match format '%y%m%d%H'. The N8 value fell through to the generic
  '%y%m%d%H%M%S'[:len(value)] branch, which parsed the four-digit year as YY.
• info('7251198007151430') raised ValueError because the 12-digit N12
  value was treated as two YYMMDD dates and '198007' is not a valid date.
• encode({'7250': datetime.date(1980, 7, 15)}) raised
  ValueError: unsupported format: N8.

_encode_date() now formats N8 as %Y%m%d and N12 as %Y%m%d%H%M, and
_decode_date() parses them back with the matching four-digit-year formats.
N12 is also removed from the two-date branch: the only date AI that encodes a
pair of YYMMDD dates is 7007 (N6[+N6]), whereas 7251 is a single datetime.

Round-trip doctests for both AIs are added to tests/test_gs1_128.doctest.

[gaoflow]

Updated the branch to pin mypy<2 in the tox mypy environment. The failing CI was installing mypy 2.1, which no longer supports the existing --python-version 3.9 target in this project. This keeps the current 3.9 type-check target working without changing runtime code.

Local verification:

• uvx --python 3.13 tox -e mypy --skip-missing-interpreters false
• uvx --python 3.13 tox -e flake8 --skip-missing-interpreters false
• uvx --python 3.13 --with pytest --with pytest-cov pytest --doctest-modules --doctest-glob='*.doctest' stdnum/gs1_128.py tests/test_gs1_128.doctest

[gaoflow]

Rebased on master; I dropped my tox.ini mypy pin from this branch since you already capped mypy the same way in b99cfad, so the PR is now just the GS1-128 change.