Added a link to the Python SDK (#82)

Author: regenara

content/4.developer/2.client-libraries.md

@@ -17,6 +17,7 @@ There are a number of client libraries available to help send e-mail using the P
 * [Go](https://github.com/Pacerino/postal-go)
 * [Elixir (via Swoosh)](https://hexdocs.pm/swoosh/Swoosh.Adapters.Postal.html)
 * [Drupal](https://www.drupal.org/project/postal_mail)
+* [Python](https://github.com/regenara/postal_py)
 
 All of these libraries will make use of the API rather than using any SMTP protocol - this is considered to be best approach for delivering your messages.