chore(python): update synthtool commit identifier to drop python 3.9 from README (#6111)

This updates the synthtool reference to a commit that includes the most
recent change to the README text.

The change in the README text drops Python 3.9 as a supported runtime.

---------

Signed-off-by: Chalmer Lowe <chalmerlowe@google.com>
Co-authored-by: Anthonios Partheniou <partheniou@google.com>

Author: chalmerlowe

internal/librarian/python/librarian.yaml

@@ -30,5 +30,5 @@ tools:
       version: "23.7.0"
       package: "black[jupyter]==23.7.0"
     - name: synthtool
-      version: "96f416c959fbe8048200b6c16000de32b352902e"
-      package: "gcp-synthtool@git+https://github.com/googleapis/synthtool@96f416c959fbe8048200b6c16000de32b352902e"
+      version: "e643ce8e20f8fe237a31a1754524ba987de72875"
+      package: "gcp-synthtool@git+https://github.com/googleapis/synthtool@e643ce8e20f8fe237a31a1754524ba987de72875"