docs: show that Python 3.9 and 3.10 are supported in classifiers (#351)

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/python-bigquery-storage/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Fixes #342 🦕

Author: tswast

packages/google-cloud-bigquery-storage/.github/sync-repo-settings.yaml

@@ -13,3 +13,5 @@ branchProtectionRules:
     - 'Samples - Python 3.6'
     - 'Samples - Python 3.7'
     - 'Samples - Python 3.8'
+    - 'Samples - Python 3.9'
+    - 'Samples - Python 3.10'

packages/google-cloud-bigquery-storage/setup.py

@@ -72,6 +72,8 @@
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
+        "Programming Language :: Python :: 3.10",
         "Operating System :: OS Independent",
         "Topic :: Internet",
     ],