Increment python debugger version in preparation for releasing 3.7.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=208671429

Author: xinghuadou-google

src/googleclouddebugger/version.py

@@ -4,4 +4,4 @@
 # The major version should only change on breaking changes. Minor version
 # changes go between regular updates. Instances running debuggers with
 # different major versions will show up as two different debuggees.
-__version__ = '2.8'
+__version__ = '2.9'