Bump up minimum version of python to 3.10. (#21307)

The latest versions of JAX and TensorFlow will no longer support
Python 3.9.  They do now support 3.12.

Author: cantonios

CONTRIBUTING.md

@@ -83,7 +83,7 @@ To set up your local dev environment, you will need the following tools.
 2.  [python](https://www.python.org/) to build and code in Keras.
 
 The following commands check the tools above are successfully installed. Note
-that Keras requires at least Python 3.9 to run.
+that Keras requires at least Python 3.10 to run.
 
 ```shell
 git --version

pyproject.toml

@@ -9,13 +9,12 @@ authors = [
 ]
 description = "Multi-backend Keras"
 readme = "README.md"
-requires-python = ">=3.9"
+requires-python = ">=3.10"
 license = {text = "Apache License 2.0"}
 dynamic = ["version"]
 classifiers = [
     "Development Status :: 4 - Beta",
     "Programming Language :: Python :: 3",
-    "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3.10",
     "Programming Language :: Python :: 3.11",
     "Programming Language :: Python :: 3 :: Only",