Update readme to indicate support for Python 3.9
Update readme to indicate support for Python 3.9

PiperOrigin-RevId: 367657091
Change-Id: I2b66dac0f86688cac47410c6954354e593e6fab9

GoogleCloudPlatform/cloud-debug-python,GoogleCloudPlatform/cloud-debug-python,GoogleCloudPlatform/cloud-debug-python