Bump version (#2618)

Author: hekaisheng

mars/_version.py

@@ -16,7 +16,7 @@
 import os
 from typing import NamedTuple, Optional
 
-version_info = (0, 8, 0, "rc1")
+version_info = (0, 9, 0, "a1")
 _num_index = max(idx if isinstance(v, int) else 0 for idx, v in enumerate(version_info))
 __version__ = ".".join(map(str, version_info[: _num_index + 1])) + "".join(
     version_info[_num_index + 1 :]