Separate the various parts of the error report with newlines (#11659)

Previously the error report would have all sections glued together:

- The assertion representation
- The error explanation
- The full diff

This makes it hard to see at a glance where which one starts and ends.

One of the representation (dataclasses, tuples, attrs) does display a
newlines at the start already.

Let's add a newlines before the error explanation and before the full
diff, so we get an easier to read report.

This has one disadvantage: we get one line less in the least verbose
mode, where the output gets truncated.

src/_pytest/assertion/util.py

@@ -230,6 +230,8 @@ def assertrepr_compare(
     if not explanation:
         return None
 
+    if explanation[0] != "":
+        explanation = [""] + explanation
     return [summary] + explanation
 
 
@@ -332,7 +334,7 @@ def _compare_eq_iterable(
     left_formatting = PrettyPrinter().pformat(left).splitlines()
     right_formatting = PrettyPrinter().pformat(right).splitlines()
 
-    explanation = ["Full diff:"]
+    explanation = ["", "Full diff:"]
     # "right" is the expected base against which we compare "left",
     # see https://github.com/pytest-dev/pytest/issues/3333
     explanation.extend(