Remove newlines from left/right operands with '-vv' (#9743)

The left/right operands produced when `verbose > 1` should not contain newlines, because they are used to 
build the `summary` string. The `assertrepr_compare` function returns a list of lines, and the summary is one of those lines and should not contain newlines itself. 

Fix #9742

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>

testing/test_assertion.py

@@ -1695,3 +1695,18 @@ def test_assertion_location_with_coverage(pytester: Pytester) -> None:
             "*= 1 failed in*",
         ]
     )
+
+
+def test_reprcompare_verbose_long() -> None:
+    a = {f"v{i}": i for i in range(11)}
+    b = a.copy()
+    b["v2"] += 10
+    lines = callop("==", a, b, verbose=2)
+    assert lines is not None
+    assert lines[0] == (
+        "{'v0': 0, 'v1': 1, 'v2': 2, 'v3': 3, 'v4': 4, 'v5': 5, "
+        "'v6': 6, 'v7': 7, 'v8': 8, 'v9': 9, 'v10': 10}"
+        " == "
+        "{'v0': 0, 'v1': 1, 'v2': 12, 'v3': 3, 'v4': 4, 'v5': 5, "
+        "'v6': 6, 'v7': 7, 'v8': 8, 'v9': 9, 'v10': 10}"
+    )