bpo-39681: Fix C pickle regression with minimal file-like objects (#18592)

Fix a regression where the C pickle module wouldn't allow unpickling from a
file-like object that doesn't expose a readinto() method.

Lib/test/pickletester.py

@@ -73,6 +73,18 @@ class UnseekableIO(io.BytesIO):
         raise io.UnsupportedOperation
 
 
+class MinimalIO(object):
+    """
+    A file-like object that doesn't support readinto().
+    """
+    def __init__(self, *args):
+        self._bio = io.BytesIO(*args)
+        self.getvalue = self._bio.getvalue
+        self.read = self._bio.read
+        self.readline = self._bio.readline
+        self.write = self._bio.write
+
+
 # We can't very well test the extension registry without putting known stuff
 # in it, but we have to be careful to restore its original state.  Code
 # should do this:
@@ -3363,7 +3375,7 @@ class AbstractPicklerUnpicklerObjectTests(unittest.TestCase):
         f.seek(0)
         self.assertEqual(unpickler.load(), data2)
 
-    def _check_multiple_unpicklings(self, ioclass):
+    def _check_multiple_unpicklings(self, ioclass, *, seekable=True):
         for proto in protocols:
             with self.subTest(proto=proto):
                 data1 = [(x, str(x)) for x in range(2000)] + [b"abcde", len]
@@ -3376,10 +3388,10 @@ class AbstractPicklerUnpicklerObjectTests(unittest.TestCase):
                 f = ioclass(pickled * N)
                 unpickler = self.unpickler_class(f)
                 for i in range(N):
-                    if f.seekable():
+                    if seekable:
                         pos = f.tell()
                     self.assertEqual(unpickler.load(), data1)
-                    if f.seekable():
+                    if seekable:
                         self.assertEqual(f.tell(), pos + len(pickled))
                 self.assertRaises(EOFError, unpickler.load)
 
@@ -3387,7 +3399,12 @@ class AbstractPicklerUnpicklerObjectTests(unittest.TestCase):
         self._check_multiple_unpicklings(io.BytesIO)
 
     def test_multiple_unpicklings_unseekable(self):
-        self._check_multiple_unpicklings(UnseekableIO)
+        self._check_multiple_unpicklings(UnseekableIO, seekable=False)
+
+    def test_multiple_unpicklings_minimal(self):
+        # File-like object that doesn't support peek() and readinto()
+        # (bpo-39681)
+        self._check_multiple_unpicklings(MinimalIO, seekable=False)
 
     def test_unpickling_buffering_readline(self):
         # Issue #12687: the unpickler's buffering logic could fail with