feat: add support for Python 3.9 (#111)

Also, add missing declaration of support for Python 3.8.

Closes #110.
diff --git a/setup.py b/setup.py
index c8fd257..baea1c0 100644
--- a/setup.py
+++ b/setup.py
@@ -53,7 +53,7 @@
 version = {}
 with open(os.path.join(package_root, "google/api_core/version.py")) as fp:
     exec(fp.read(), version)
-version = version['__version__']
+version = version["__version__"]
 
 readme_filename = os.path.join(package_root, "README.rst")
 with io.open(readme_filename, encoding="utf-8") as readme_file:
@@ -91,6 +91,8 @@
         "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
+        "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
         "Operating System :: OS Independent",
         "Topic :: Internet",
     ],