Adds tests to test reading of legacy files

tests/data/archives/README.md

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+# Archives with legacy test data
+
+These archives should contain folders containing `.h5mu` test files
+creasted with older `mudata` versions for regression testing.
+
+## Format
+
+A folder with the format `v<version>` indicating the `mudata` version used
+to create the file.
+
+Containing:
+- `create_testfiles.py`: a script creating the testfiles. Should contain a
+    uv header specifying the critical dependencies. Can be run with `uv run create_testfile.py`
+    to create the testfile(s)
+- `mudata.h5mu`: the created testfile. Needs to contain at minimum the data expected by
+    `test_io_backwards_compat.py`. 
+
+## Automated tests
+
+The test `test_io_backwards_compat.py` will check each `mudata.h5mu` file in this folder if it can
+be loaded correctly by the current `mudata` version.

tests/data/archives/v0.1.2/create_testfiles.py

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+# /// script
+# requires-python = ">=3.10"
+# dependencies = [
+#     "mudata==0.1.2",
+#     "numpy==1.*", # required as numpy 2 raises errors
+# ]
+# [tool.uv]
+# exclude-newer = "2024-12-01T00:00:00Z"
+# ///
+import os
+
+import numpy as np
+from anndata import AnnData
+
+from mudata import MuData
+
+
+def create_mudata() -> MuData:
+    """Create a mudata testfile
+
+    Returns:
+        MuData: MuData object with two AnnData objects
+    """
+    return MuData(
+        {
+            "mod1": AnnData(np.arange(0, 100, 0.1).reshape(-1, 10)),
+            "mod2": AnnData(np.arange(101, 2101, 1).reshape(-1, 20)),
+        }
+    )
+
+
+def write_h5mu_testfile(mdata: MuData, filepath_h5mu: str | os.PathLike) -> None:
+    """Write a MuData object to a file
+
+    Args:
+        mdata (MuData): MuData object to write
+        filepath_h5mu (str | os.PathLike): Path to write the MuData object
+
+    Returns:
+        None
+    """
+    mdata.write(filepath_h5mu)
+
+
+if __name__ == "__main__":
+    mdata = create_mudata()
+    write_h5mu_testfile(mdata, "mudata.h5mu")

tests/test_io_backwards_compat.py

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+from __future__ import annotations
+
+from pathlib import Path
+
+import numpy as np
+import pytest
+from anndata import AnnData
+from anndata.tests.helpers import assert_equal
+
+from mudata import MuData, read_h5mu
+
+ARCHIVE_PTH = Path(__file__).parent / "data/archives"
+
+
+@pytest.fixture(params=list(ARCHIVE_PTH.glob("v*")), ids=lambda x: x.name)
+def archive_dir(request):
+    return request.param
+
+
+@pytest.fixture
+def mdata():
+    return MuData(
+        {
+            "mod1": AnnData(np.arange(0, 100, 0.1).reshape(-1, 10)),
+            "mod2": AnnData(np.arange(101, 2101, 1).reshape(-1, 20)),
+        }
+    )
+
+
+class TestLegacyMuData:
+    def test_legacy_files(self, archive_dir, mdata):
+        from_h5mu = read_h5mu(archive_dir / "mudata.h5mu")
+
+        for mod in mdata.mod:
+            assert_equal(from_h5mu.mod[mod].X, mdata.mod[mod].X, exact=False)