bpo-39689: Do not test undefined casts to _Bool (GH-18964) (#18965)

- When casting to _Bool, arrays should only contain zeros or ones.
(cherry picked from commit 1ae9cde4b2)

Co-authored-by: Stefan Krah <skrah@bytereef.org>

Lib/test/test_buffer.py

@@ -2730,6 +2730,10 @@ class TestBufferProtocol(unittest.TestCase):
         # be 1D, at least one format must be 'c', 'b' or 'B'.
         for _tshape in gencastshapes():
             for char in fmtdict['@']:
+                # Casts to _Bool are undefined if the source contains values
+                # other than 0 or 1.
+                if char == "?":
+                    continue
                 tfmt = ('', '@')[randrange(2)] + char
                 tsize = struct.calcsize(tfmt)
                 n = prod(_tshape) * tsize