docs: update python version in readme

Author: jolars

README.rst

@@ -78,8 +78,8 @@ Use `benchopt run -h` for more details about these options, or visit https://ben
 
 .. |Build Status| image:: https://github.com/benchopt/benchmark_slope/workflows/Tests/badge.svg
    :target: https://github.com/benchopt/benchmark_slope/actions
-.. |Python 3.6+| image:: https://img.shields.io/badge/python-3.6%2B-blue
-   :target: https://www.python.org/downloads/release/python-360/
+.. |Python 3.9+| image:: https://img.shields.io/badge/python-3.9%2B-blue
+   :target: https://www.python.org/downloads/release/python-3925/
 
 Acknowledgments
 ---------------