contrib: Fix SyntaxWarning in Python base58 implementation

In Python integers should be compared for equality (`i == j`), not identity (`i is j`). Recent versions of CPython 3.x emit a SyntaxWarning when they encounter this incorrect usage, e.g.

```
$ python3 base58.py 
base58.py:110: SyntaxWarning: "is" with a literal. Did you mean "=="?
  assert get_bcaddress_version('15VjRaDX9zpbA8LVnbrCAFzrVzN7ixHNsC') is 0
Tests passed
```

contrib/testgen/base58.py

@@ -107,7 +107,7 @@ def get_bcaddress_version(strAddress):
 
 if __name__ == '__main__':
     # Test case (from http://gitorious.org/bitcoin/python-base58.git)
-    assert get_bcaddress_version('15VjRaDX9zpbA8LVnbrCAFzrVzN7ixHNsC') is 0
+    assert get_bcaddress_version('15VjRaDX9zpbA8LVnbrCAFzrVzN7ixHNsC') == 0
     _ohai = 'o hai'.encode('ascii')
     _tmp = b58encode(_ohai)
     assert _tmp == 'DYB3oMS'