_compare_eq_iterable: use AlwaysDispatchingPrettyPrinter

This fixes/removes the previous hack of re-trying with minimum width,
which fails short when it splits strings.

This inherits from `pprint.PrettyPrinter` to override `_format` in a
minimal way to always dispatch, regardless of the given width.

Code ref: 5c0c325453/Lib/pprint.py (L170-L178)

src/_pytest/assertion/util.py

@@ -28,6 +28,27 @@ _reprcompare = None  # type: Optional[Callable[[str, object, object], Optional[s
 _assertion_pass = None  # type: Optional[Callable[[int, str, str], None]]
 
 
+class AlwaysDispatchingPrettyPrinter(pprint.PrettyPrinter):
+    """PrettyPrinter that always dispatches (regardless of width)."""
+
+    def _format(self, object, stream, indent, allowance, context, level):
+        p = self._dispatch.get(type(object).__repr__, None)
+
+        objid = id(object)
+        if objid in context or p is None:
+            return super()._format(object, stream, indent, allowance, context, level)
+
+        context[objid] = 1
+        p(self, object, stream, indent, allowance, context, level + 1)
+        del context[objid]
+
+
+def _pformat_dispatch(object, indent=1, width=80, depth=None, *, compact=False):
+    return AlwaysDispatchingPrettyPrinter(
+        indent=1, width=80, depth=None, compact=False
+    ).pformat(object)
+
+
 def format_explanation(explanation: str) -> str:
     """This formats an explanation
 
@@ -270,15 +291,8 @@ def _compare_eq_iterable(
     lines_left = len(left_formatting)
     lines_right = len(right_formatting)
     if lines_left != lines_right:
-        if lines_left > lines_right:
-            max_width = min(len(x) for x in left_formatting)
-        else:
-            max_width = min(len(x) for x in right_formatting)
-
-        right_formatting = pprint.pformat(right, width=max_width).splitlines()
-        lines_right = len(right_formatting)
-        left_formatting = pprint.pformat(left, width=max_width).splitlines()
-        lines_left = len(left_formatting)
+        left_formatting = _pformat_dispatch(left).splitlines()
+        right_formatting = _pformat_dispatch(right).splitlines()
 
     if lines_left > 1 or lines_right > 1:
         _surrounding_parens_on_own_lines(left_formatting)