release 1.0.21

I forgot to update the version number in setup.py, so bump version to 1.0.21 to make sure that this correct and complete release is actually recorded in a commit.
diff --git a/ipaddress.py b/ipaddress.py
index 18001de..4bd5a31 100644
--- a/ipaddress.py
+++ b/ipaddress.py
@@ -14,7 +14,7 @@
 import itertools
 import struct
 
-__version__ = '1.0.20'
+__version__ = '1.0.21'
 
 # Compatibility functions
 _compat_int_types = (int,)
diff --git a/setup.py b/setup.py
index d01ac63..14c4301 100644
--- a/setup.py
+++ b/setup.py
@@ -8,7 +8,7 @@
 
 settings = {
     'name': 'ipaddress',
-    'version': '1.0.19',
+    'version': '1.0.21',
     'description': 'IPv4/IPv6 manipulation library',
     'long_description': 'Port of the 3.3+ ipaddress module to 2.6, 2.7, 3.2',
     'author': 'Philipp Hagemeister',