Break ZODB dependency.

BTrees/utils.py

+# Copied from ZODB/utils.py
+
+from binascii import hexlify
+from struct import calcsize
+
+_ADDRESS_MASK = 256 ** calcsize('P')
+
+def positive_id(obj):
+    """Return id(obj) as a non-negative integer."""
+
+    result = id(obj)
+    if result < 0:
+        result += _ADDRESS_MASK
+        assert result > 0
+    return result
+
+def oid_repr(oid):
+    if isinstance(oid, str) and len(oid) == 8:
+        # Convert to hex and strip leading zeroes.
+        as_hex = hexlify(oid).lstrip('0')
+        # Ensure two characters per input byte.
+        if len(as_hex) & 1:
+            as_hex = '0' + as_hex
+        elif as_hex == '':
+            as_hex = '00'
+        return '0x' + as_hex
+    else:
+        return repr(oid)