Drop Python 3.8 support for TFP 0.22 and later.

Author: jburnim

setup.py

@@ -70,7 +70,7 @@ def has_ext_modules(self):
     url='http://github.com/tensorflow/probability',
     license='Apache 2.0',
     packages=find_packages(),
-    python_requires='>=3.8',
+    python_requires='>=3.9',
     install_requires=REQUIRED_PACKAGES,
     # Add in any packaged data.
     include_package_data=True,