Add method to read db file

repo_management/files.py:
Add `_read_db_file()` to open a db file and return it as
tarfile.Tarfile. The method assumes gzip-compressed databases by
default, but can be used with other compression algorithms.

tests/test_file.py:
Add fixtures and tests for `_read_db_file()`.

repo_management/files.py

+import tarfile
+from pathlib import Path
+
+
+def _read_db_file(db_path: Path, compression: str = "gz") -> tarfile.TarFile:
+    """Read a repository database file
+
+    Parameters
+    ----------
+    db_path: Path
+        A pathlib.Path instance, representing the location of the database file
+    compression: str
+        The compression used for the database file (defaults to 'gz')
+
+    Raises
+    ------
+    ValueError
+        If the file represented by db_path does not exist
+    tarfile.ReadError
+        If the file could not be opened
+    tarfile.CompressionError
+        If the provided compression does not match the compression of the file or if the compression type is unknown
+
+    Returns
+    -------
+    tarfile.Tarfile
+        An instance of Tarfile
+    """
+
+    return tarfile.open(name=db_path, mode=f"r:{compression}")

tests/test_file.py

+import os
+import shutil
+import tarfile
+import tempfile
+from contextlib import nullcontext as does_not_raise
+from pathlib import Path
+from typing import Iterator
+
+from pytest import fixture, raises
+
+from repo_management import files
+
+
+def create_db_file(compression: str = "gz", remove_db: bool = False) -> Path:
+    (file_number, db_file) = tempfile.mkstemp(suffix=".db")
+    temp_dirs = [
+        tempfile.mkdtemp(),
+        tempfile.mkdtemp(),
+        tempfile.mkdtemp(),
+        tempfile.mkdtemp(),
+    ]
+    with tarfile.open(db_file, f"w:{compression}") as db_tar:
+        for name in temp_dirs:
+            db_tar.add(name)
+    for name in temp_dirs:
+        shutil.rmtree(name)
+    if remove_db:
+        os.remove(db_file)
+
+    return Path(db_file)
+
+
+@fixture(scope="function")
+def create_gz_db_file() -> Iterator[Path]:
+    db_file = create_db_file()
+    yield db_file
+    os.remove(db_file)
+
+
+@fixture(scope="function")
+def create_bzip_db_file() -> Iterator[Path]:
+    db_file = create_db_file(compression="bz2")
+    yield db_file
+    os.remove(db_file)
+
+
+@fixture(scope="function")
+def create_null_db_file() -> Iterator[Path]:
+    yield create_db_file(remove_db=True)
+
+
+def test__read_db_file(create_gz_db_file: Path) -> None:
+    with does_not_raise():
+        files._read_db_file(create_gz_db_file)
+
+
+def test__read_db_file_wrong_compression(create_gz_db_file: Path) -> None:
+    with raises(tarfile.CompressionError):
+        files._read_db_file(create_gz_db_file, compression="foo")
+
+
+def test__read_db_file_does_not_exist(create_null_db_file: Path) -> None:
+    with raises(FileNotFoundError):
+        files._read_db_file(create_null_db_file)
+
+
+def test__read_db_file_wrong_db_compression(create_bzip_db_file: Path) -> None:
+    with raises(tarfile.ReadError):
+        files._read_db_file(create_bzip_db_file)