[3.11] gh-74678: Increase base64 test coverage (GH-21913) (GH-113810)

Ensure the character y is disallowed within an Ascii85 5-tuple.

(cherry picked from commit 802d495)

Co-authored-by: Zackery Spytz <[email protected]>
Co-authored-by: Lee Cannon <[email protected]>

Author: 3 people

Lib/test/test_base64.py

@@ -582,6 +582,7 @@ def test_a85decode(self):
         eq(base64.a85decode(b'y+<Vd', foldspaces=True, adobe=False), b' '*7)
         eq(base64.a85decode(b'y+<U', foldspaces=True, adobe=False), b' '*6)
         eq(base64.a85decode(b'y+9', foldspaces=True, adobe=False), b' '*5)
+        eq(base64.a85decode(b'aaaaay', foldspaces=True), b'\xc9\x80\x0b@    ')
 
         self.check_other_types(base64.a85decode, b'GB\\6`E-ZP=Df.1GEb>',
                                b"www.python.org")
@@ -685,6 +686,8 @@ def test_a85decode_errors(self):
         self.assertRaises(ValueError, base64.a85decode, b's8W', adobe=False)
         self.assertRaises(ValueError, base64.a85decode, b's8W-', adobe=False)
         self.assertRaises(ValueError, base64.a85decode, b's8W-"', adobe=False)
+        self.assertRaises(ValueError, base64.a85decode, b'aaaay',
+                          foldspaces=True)
 
     def test_b85decode_errors(self):
         illegal = list(range(33)) + \