Refine Python version constraint in readme example

Author: bbc2

README.md

@@ -84,7 +84,7 @@ load_dotenv()
 load_dotenv(verbose=True)
 
 # OR, explicitly providing path to '.env'
-from pathlib import Path  # python3 only
+from pathlib import Path  # Python 3.6+ only
 env_path = Path('.') / '.env'
 load_dotenv(dotenv_path=env_path)
 ```