Update requirements to run SAPP

Summary:
Add documentation to inform users they need Python 3.7 or later to run SAPP, because we use [`from __future__ import annotations`](https://github.com/facebook/sapp/blob/master/sapp/ui/interactive.py#L6), which is [only enabled in Python 3.7 onwards](https://www.python.org/dev/peps/pep-0563/#enabling-the-future-behavior-in-python-3-7)

## Steps to Reproduce
1. Install a Python version lower than 3.7
2. `python3 -m venv ~/.venvs/sapp-py-version-req`
3. `source ~/.venvs/sapp-py-version-req/bin/activate`
4. `(sapp-py-version-req) python -v` to ensure the python version is lower than 3.7
5. `(sapp-py-version-req) pip install fb-sapp`
6. `(sapp-py-version-req) sapp server`
7. You should get a traceback with `SyntaxError: future feature annotations is not defined`

Reviewed By: gbleaney

Differential Revision: D27640126

fbshipit-source-id: a397d680b0e7f290a555aba8480c9d67933f4d81

README.md

@@ -6,7 +6,7 @@
 SAPP stands for Static Analysis Post Processor. SAPP takes the raw results of Pysa and makes them explorable both through a command line interface and a web UI.
 
 ## Installation
-SAPP can be installed through [PyPI](https://pypi.org/) with `pip install fb-sapp`.
+To run SAPP, you will need [Python 3.7 or later](https://www.python.org/getit/). SAPP can be installed through [PyPI](https://pypi.org/) with `pip install fb-sapp`.
 
 ## Getting Started
 This guide assumes that you have results from a Pysa run saved in a `~/example` directory. If you are new to Pysa, you can follow [this tutorial](https://pyre-check.org/docs/pysa-running#example) to get started.