Made the mistake to release from GitHub while this must be done in code.

onekiloparsec · onekiloparsec · commit 675404c5f6b2 · 2019-12-14T15:43:44.000+01:00
Hence the jump in two versions.
diff --git a/arcsecond/__init__.py b/arcsecond/__init__.py
@@ -7,4 +7,4 @@
            "ArcsecondConnectionError",
            "ArcsecondInvalidEndpointError"]
 
-__version__ = '0.6.4'
+__version__ = '0.6.6'
