Release 1.0a20

Refs #2488, #2495, #2503, #2505, #2509, #2510, #2513, #2515, #2517, #2519, #2520, #2521,
#2524, #2525, #2526, #2528, #2530, #2531, #2534, #2537, #2543, #2544, #2550, #2551,
#2555, #2558, #2561, #2562, #2564, #2565, #2567, #2569, #2570, #2571, #2574

datasette/version.py

@@ -1,2 +1,2 @@
-__version__ = "1.0a19"
+__version__ = "1.0a20"
 __version_info__ = tuple(__version__.split("."))