remove sixpack: inactive with security vulnerabilities

No new releases in 12+ months, 19 known vulnerabilities.
Consider modern A/B testing solutions instead.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: vinta

README.md

@@ -1041,7 +1041,6 @@ Inspired by [awesome-php](https://github.com/ziadoz/awesome-php).
     * [PyAutoGUI](https://github.com/asweigart/pyautogui) - PyAutoGUI is a cross-platform GUI automation Python module for human beings.
     * [Schemathesis](https://github.com/kiwicom/schemathesis) - A tool for automatic property-based testing of web applications built with Open API / Swagger specifications.
     * [Selenium](https://pypi.org/project/selenium/) - Python bindings for [Selenium](https://selenium.dev/) [WebDriver](https://selenium.dev/documentation/webdriver/).
-    * [sixpack](https://github.com/seatgeek/sixpack) - A language-agnostic A/B Testing framework.
     * [splinter](https://github.com/cobrateam/splinter) - Open source tool for testing web applications.
 * Mock
     * [doublex](https://pypi.org/project/doublex/) - Powerful test doubles framework for Python.