Support Python 3.9

[freelerobot]

Renovate bot upgraded our docker images to Python:3.9

google-python-cloud-debugger==2.15 is only compatible with 3.7/3.8.

Is it possible to upgrade compatibility with Python 3.9 as well?

Setting google-python-cloud-debugger==2.15 as a requirement yields:

ERROR: Could not find a version that satisfies the requirement google-python-cloud-debugger==2.15 (from -r requirements.txt (line 17)) (from versions: none)
ERROR: No matching distribution found for google-python-cloud-debugger==2.15 (from -r requirements.txt (line 17))

[di]

I'll add that since the last release, we also have manylinux2010 and manylinux2014, the latter of which supports additional architectures including ARM, which would help resolve #12.

[jturmel]

Any idea when this will push out? Now that Python 3.9 is GA for App Engine it makes upgrading to 3.9 in production a no-go because we rely on Cloud Debugger.

[mctavish]

Support for Python 3.9 has been added in https://github.com/GoogleCloudPlatform/cloud-debug-python/releases/tag/v2.16 and is available at https://pypi.org/project/google-python-cloud-debugger/2.16/

Let me know if you run into any problems!