Increment Python agent version.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=192156908

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__ = '2.7'
+__version__ = '2.8'