Minimize memory consuption of _dlnode

Added `__slots__` do `_dlnode` class (which saves 88 bytes per one object). 

Before this change, each _dlnode () object was consuming 168 bytes:

```python
from sys import getsizeof
d = _dlnode()
d.next = d.prev = d.key = d.value = None
assert getsizeof(d) + getsizeof(d.__dict__) == 168
```

After this change each _dlnode () object is consuming 80 bytes:

```python
from sys import getsizeof
d = _dlnode()
d.next = d.prev = d.key = d.value = None
assert getsizeof(d) == 80
```

1 files changed, 2 insertions, 0 deletions

diff --git a/pylru.py b/pylru.py
index c631e27..b3c321a 100644
--- a/pylru.py
+++ b/pylru.py
@@ -39,6 +39,8 @@ else:
 
 # Class for the node objects.
 class _dlnode(object):
+    __slots__ = ('empty', 'next', 'prev', 'key', 'value')
+
     def __init__(self):
         self.empty = True