Update pyproject.toml

Changed python requirements, if the future constrain is there other project with less strict requirements will fail to install river,

for instance i have in my pyproject.toml python = "^3.11" and fails with "requires Python <3.13,>=3.9, river is forbidden."

pyproject.toml

@@ -28,7 +28,7 @@ generate-setup-file = true
 script = "build.py"
 
 [tool.poetry.dependencies]
-python = ">=3.9,<3.13"
+python = "^3.9"
 numpy = "^1.23.0"
 scipy = "^1.8.1"
 pandas = "^2.1"