Revert "removed useless doctest for AC4 in Sudoku because AC4's tests are already present in test_csp.py"

This reverts commit b3cd24c.

Author: dmeoli

csp.py

@@ -673,6 +673,36 @@ class Sudoku(CSP):
     >>> h = Sudoku(harder1)
     >>> backtracking_search(h, select_unassigned_variable=mrv, inference=forward_checking) is not None
     True
+
+    >>> e = Sudoku(easy1)
+    >>> e.display(e.infer_assignment())
+    . . 3 | . 2 . | 6 . .
+    9 . . | 3 . 5 | . . 1
+    . . 1 | 8 . 6 | 4 . .
+    ------+-------+------
+    . . 8 | 1 . 2 | 9 . .
+    7 . . | . . . | . . 8
+    . . 6 | 7 . 8 | 2 . .
+    ------+-------+------
+    . . 2 | 6 . 9 | 5 . .
+    8 . . | 2 . 3 | . . 9
+    . . 5 | . 1 . | 3 . .
+    >>> AC4(e); e.display(e.infer_assignment())
+    True
+    4 8 3 | 9 2 1 | 6 5 7
+    9 6 7 | 3 4 5 | 8 2 1
+    2 5 1 | 8 7 6 | 4 9 3
+    ------+-------+------
+    5 4 8 | 1 3 2 | 9 7 6
+    7 2 9 | 5 6 4 | 1 3 8
+    1 3 6 | 7 9 8 | 2 4 5
+    ------+-------+------
+    3 7 2 | 6 8 9 | 5 1 4
+    8 1 4 | 2 5 3 | 7 6 9
+    6 9 5 | 4 1 7 | 3 8 2
+    >>> h = Sudoku(harder1)
+    >>> backtracking_search(h, select_unassigned_variable=mrv, inference=forward_checking) is not None
+    True
     """
 
     R3 = _R3