Escape whitespace only strings when diffing them on failed assertions

Fix #3443

testing/test_assertion.py

@@ -746,6 +746,18 @@ def test_reprcompare_notin(mock_config):
     assert detail == ["'foo' is contained here:", '  aaafoobbb', '?    +++']
 
 
+def test_reprcompare_whitespaces(mock_config):
+    detail = plugin.pytest_assertrepr_compare(
+        mock_config, '==', '\r\n', '\n')
+    assert detail == [
+        r"'\r\n' == '\n'",
+        r"Strings contain only whitespace, escaping them using repr()",
+        r"- '\r\n'",
+        r"?  --",
+        r"+ '\n'",
+    ]
+
+
 def test_pytest_assertrepr_compare_integration(testdir):
     testdir.makepyfile("""
         def test_hello():