doc: Require Python3.8 to sapp (#787)

Summary:
**Pre-submission checklist**
- [x] I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running `pip install -r requirements-dev.txt && pre-commit install`
- [x] `pre-commit run`

`sapp` require Python3.8 since facebook/sapp@27b4220

Pull Request resolved: #787

Test Plan: N/A

Reviewed By: grievejia

Differential Revision: D49514635

Pulled By: alexkassil

fbshipit-source-id: 4ce3f36c56206b9a43d56378f51c82146b715afc

documentation/website/docs/pysa_quickstart.md

@@ -230,7 +230,7 @@ If you do use the `module.path.function_name` mentioned in your project and the
 ----
 **Problem**: Running any `sapp` command results in `SyntaxError: future feature annotations is not defined`
 
-**Solution**: SAPP requires Python 3.7. Ensure you are running a Python version later than Python 3.7
+**Solution**: SAPP requires Python 3.8. Ensure you are running a Python version later than Python 3.8
 ```shell
 $ python3 --version
 ```