Use operator.index to avoid DeprecationWarning on Python 2

Fixes #79

Slightly faster on CPython:

$ python -m perf timeit -s 'from struct import Struct; s = Struct("i"); unpack=s.unpack; pack=s.pack' 'unpack(pack(123456))'
.....................
Mean +- std dev: 252 ns +- 12 ns
$ python -m perf timeit -s 'from struct import Struct; s = Struct("i"); from operator import index; pack=s.pack' 'pack(index(123456))'
.....................
Mean +- std dev: 235 ns +- 6 ns

Very slightly faster on PyPy:

python -m perf timeit -s 'from struct import Struct; s = Struct("i"); unpack=s.unpack; pack=s.pack' 'unpack(pack(123456))'
.........
Mean +- std dev: 4.24 ns +- 0.12 ns
$ python -m perf timeit -s 'from struct import Struct; s = Struct("i"); from operator import index; pack=s.pack' 'pack(index(123456))'
.........
Mean +- std dev: 4.19 ns +- 0.10 ns

BTrees/_base.py

@@ -16,6 +16,7 @@
 
 from struct import Struct
 from struct import error as struct_error
+from operator import index
 
 from persistent import Persistent
 
@@ -1503,12 +1504,8 @@ def _packer_unpacker(struct_format):
 
 def to_int(self, v):
     try:
-        # XXX Python 2.6 doesn't truncate, it spews a warning.
-        if not int_unpack(int_pack(v))[0] == v: #pragma: no cover
-            raise TypeError('32-bit integer expected')
-    except (struct_error,
-            OverflowError, #PyPy
-           ):
+        int_pack(index(v))
+    except (struct_error, TypeError):
         raise TypeError('32-bit integer expected')
 
     return int(v)
@@ -1527,14 +1524,8 @@ def to_float(self, v):
 
 def to_long(self, v):
     try:
-        # XXX Python 2.6 doesn't truncate, it spews a warning.
-        if not long_unpack(long_pack(v))[0] == v: #pragma: no cover
-            if isinstance(v, int_types):
-                raise ValueError("Value out of range", v)
-            raise TypeError('64-bit integer expected')
-    except (struct_error,
-            OverflowError, #PyPy
-           ):
+        long_pack(index(v))
+    except (struct_error, TypeError):
         if isinstance(v, int_types):
             raise ValueError("Value out of range", v)
         raise TypeError('64-bit integer expected')

CHANGES.rst

@@ -14,7 +14,10 @@
 - Respect the ``PURE_PYTHON`` environment variable at runtime even if
   the C extensions are available. See
   https://github.com/zopefoundation/BTrees/issues/78
-- Always attempt to build the C extensions, but make their success optional.
+- Always attempt to build the C extensions, but make their success
+  optional.
+- Fix a ``DeprecationWarning`` that could come from I and L objects in
+  Python 2 in pure-Python mode. See https://github.com/zopefoundation/BTrees/issues/79
 
 4.4.1 (2017-01-24)
 ------------------