Update required python version to >= 3.9 in docs (#806)

No need to run tests - it's just a doc change

Author: matheusfelipeog

README.md

@@ -101,7 +101,7 @@ pip install pandas-stubs
 
 ## Installation from sources
 
-- Make sure you have `python >= 3.8` installed.
+- Make sure you have `python >= 3.9` installed.
 - Install poetry
 
 ```sh

docs/setup.md

@@ -1,6 +1,6 @@
 ## Set Up Environment
 
-- Make sure you have `python >= 3.8` installed.
+- Make sure you have `python >= 3.9` installed.
 - Install poetry: `pip install poetry`
 - Install the project dependencies: `poetry update`
 - Enter the virtual environment: `poetry shell`