support postgres-xl version

Author: songjy

asyncpg/serverversion.py

@@ -12,6 +12,9 @@ def split_server_version_string(version_string):
     version_string = version_string.strip()
     if version_string.startswith('PostgreSQL '):
         version_string = version_string[len('PostgreSQL '):]
+    index = version_string.find("Postgres-XL")
+    if index > 0:
+        version_string = version_string[0: index-1]
 
     parts = version_string.strip().split('.')
     if not parts[-1].isdigit():