Support reading GitHub token from env var

I would like to share my config.yaml and update scripts in a git repo, so I need a way to supply the GitHub token outside of config.yaml

This solution felt natural to me, but I would be willing to take a second pass utilizing a credentials file instead.

Thanks for this great tool!

README.md

@@ -59,6 +59,8 @@ update_command:
   - my-file
 ```
 
+If you wish to keep your API Key outside of `config.yaml`, set the env var `APR_API_KEY` with your GitHub Token
+
 ### Repositories
 
 You can define the list of repositories to pull and build into the database to update using a list of rules.

autopr/config.py

@@ -1,5 +1,6 @@
 from dataclasses import dataclass, field
 from typing import List, Optional
+from os import environ
 
 import marshmallow_dataclass
 
@@ -17,8 +18,8 @@
 
 @dataclass
 class Credentials:
-    api_key: str
     ssh_key_file: str
+    api_key: str = environ.get("APR_API_KEY", "")
 
 
 CREDENTIALS_SCHEMA = marshmallow_dataclass.class_schema(Credentials)()