Fix AttributeError in .convert.pandas with v2.23.0

[khaeru]

v2.23.0 triggered errors downstream, e.g. here:

        # Set private attributes
        self._strict = issubclass(
>           self.format_options.format, (csv.v1.FORMAT, csv.v2.FORMAT)
                                         ^^^^^^
        )
E       AttributeError: module 'sdmx.format.csv' has no attribute 'v1'. Did you mean: 'v2'?

.venv/lib/python3.13/site-packages/sdmx/convert/pandas.py:498: AttributeError

h/t @glatterf42 for spotting the issue.

This commit fixes by ensuring the submodules are available as attributes of their parent.

PR checklist

• Checks all ✅
• Update documentation Bug fix.
• Update doc/whatsnew.rst

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.25%. Comparing base (342dd7d) to head (67cbb17).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #247      +/-   ##
==========================================
- Coverage   99.02%   98.25%   -0.78%     
==========================================
  Files         114      115       +1     
  Lines        9329     9331       +2     
==========================================
- Hits         9238     9168      -70     
- Misses         91      163      +72     

Files with missing lines	Coverage Δ
sdmx/format/csv/__init__.py	100.00% <100.00%> (ø)

... and 20 files with indirect coverage changes

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