Library incompatible with python 3.9

Author: mllemango

.github/workflows/build.yml

@@ -10,7 +10,7 @@ jobs:
     name: Python ${{ matrix.version }}
     strategy: 
       matrix:
-        version: [2.7, 3.6, 3.9]
+        version: [2.7, 3.6, 3.8]
 
     steps: 
       - uses: actions/checkout@v2

README.md

@@ -10,6 +10,9 @@ The [Shopify Admin API](https://shopify.dev/docs/admin-api) Python Library
 ### Requirements
 You should be signed up as a partner on the [Shopify Partners Dashboard](https://partners.shopify.com) so that you can create and manage shopify applications.
 
+#### Python version
+This library requires Python 3.8 or lower.
+
 ### Installation
 
 To easily install or upgrade to the latest release, use [pip](http://www.pip-installer.org/).