Mention tox in the README

README.md

@@ -236,12 +236,13 @@ all_results = collect_paginated_api(
 
 ## Testing
 
-Run the tests with the `pytest` command: the tests are using pytest-vcr's cassettes
-for simulating requests to the Notion API.
+Run the tests with the `pytest` command. If you want to test against all Python
+versions, you can run `tox` instead.
 
-To create new tests or run them without cassettes, you need to set up the
-environment variables `NOTION_TOKEN` and `NOTION_TEST_PAGE_ID` (a page where
-your integration has all the capabilities enabled).
+The tests are using `pytest-vcr`'s cassettes for simulating requests to the
+Notion API. To create new tests or run them without cassettes, you need to set
+up the environment variables `NOTION_TOKEN` and `NOTION_TEST_PAGE_ID` (a page
+where your integration has all the capabilities enabled).
 
 The code will use the page at `NOTION_TEST_PAGE_ID` to generate a temporary
 environment with the Notion objects to be tested, which will be deleted