Release 0.21.0

Signed-off-by: Chris Marchbanks <[email protected]>

Author: csmarchbanks

setup.py

@@ -8,7 +8,7 @@
 
 setup(
     name="prometheus_client",
-    version="0.20.0",
+    version="0.21.0",
     author="Brian Brazil",
     author_email="[email protected]",
     description="Python client for the Prometheus monitoring system.",