Release python agent version 2.16, with Python 3.9 support

PiperOrigin-RevId: 367621582
Change-Id: I1ffa31552c54e489ef3d2677c324fe0463ebb16d

Author: mctavish

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__ = '2.15'
+__version__ = '2.16'