Update setup.py

- upgrade requirements to Python >= 3.9
- increase version to 1.5 in preparation of release

setup.py

@@ -129,7 +129,7 @@ def _parse_line(s):
 
 setuptools.setup(
     name="open_spiel",
-    version="1.4",
+    version="1.5",
     license="Apache 2.0",
     author="The OpenSpiel authors",
     author_email="open_spiel@google.com",
@@ -138,7 +138,7 @@ def _parse_line(s):
     long_description_content_type="text/markdown",
     url="https://github.com/deepmind/open_spiel",
     install_requires=_get_requirements(req_file),
-    python_requires=">=3.8",
+    python_requires=">=3.9",
     ext_modules=[CMakeExtension("pyspiel", sourcedir="open_spiel")],
     cmdclass={"build_ext": BuildExt},
     zip_safe=False,