chore: bring the test suite up to ruff's standards

Mostly automatic: empty f-strings, unused imports, formatting. The manual half
was duplicated import blocks stranded mid-file and `== None` assertions, which
become `is None` here because they compare plain attributes -- unlike the
identical rule in filters/book.py, where they build SQL.

Two unused variables are the tests never checking the disk in either
delete_files direction. Left in place under a noqa so the gap stays visible.

backend/tests/unit/test_matching.py

@@ -54,7 +54,8 @@ class TestNormalizeTitle:
         assert normalize_title("The") == "the"
 
     @pytest.mark.parametrize(
-        "title", ["Catch 22", "Fahrenheit 451", "Blade Runner 2049", "1984", "Apollo 13"]
+        "title",
+        ["Catch 22", "Fahrenheit 451", "Blade Runner 2049", "1984", "Apollo 13"],
     )
     def test_a_number_is_not_an_edition(self, title: str) -> None:
         """Edition stripping keys on the `e`; a bare number is part of the title."""
@@ -163,8 +164,13 @@ class TestNormalizeIdentifier:
 
     def test_uuids_are_refused(self) -> None:
         """Generated per build, so they only re-find what the hash check catches."""
-        assert normalize_identifier("uuid", "3f2b1c4e-1111-2222-3333-444455556666") is None
-        assert normalize_identifier("urn:uuid", "3f2b1c4e-1111-2222-3333-444455556666") is None
+        assert (
+            normalize_identifier("uuid", "3f2b1c4e-1111-2222-3333-444455556666") is None
+        )
+        assert (
+            normalize_identifier("urn:uuid", "3f2b1c4e-1111-2222-3333-444455556666")
+            is None
+        )
 
     def test_other_schemes_keep_their_own_key(self) -> None:
         assert normalize_identifier("asin", "B000FC0PDA") == "asin:b000fc0pda"
@@ -187,7 +193,9 @@ class TestIsbnConversion:
         assert isbn10_to_isbn13("043942089X") == "9780439420891"
 
     @pytest.mark.parametrize("isbn", ["0486282113", "9780486282114", "nonsense"])
-    def test_anything_that_is_not_an_isbn_10_converts_to_nothing(self, isbn: str) -> None:
+    def test_anything_that_is_not_an_isbn_10_converts_to_nothing(
+        self, isbn: str
+    ) -> None:
         assert isbn10_to_isbn13(isbn) is None
 
 
@@ -219,7 +227,7 @@ class TestParseIdentifier:
         assert parse_identifier("B000FC0PDA", "mobi-asin") == ("asin", "B000FC0PDA")
 
     def test_an_unrecognised_prefix_is_part_of_the_value(self) -> None:
-        """"http://example.com/book" is not an identifier called "http"."""
+        """ "http://example.com/book" is not an identifier called "http"."""
         assert parse_identifier("http://www.gutenberg.org/5200") == (
             "id",
             "http://www.gutenberg.org/5200",