drop Python 3.9 support

Astropy 5.0 was the first to support 3.10, so >=4.3 constraint is now superfluous

Author: JostMigenda

pyproject.toml

@@ -10,12 +10,12 @@ authors = [{ name = "SNEWS Collaboration", email = "snews2.0@lists.bnl.gov" }]
 license = { text = "BSD" }
 readme = {file = "README.md", content-type = "text/markdown"}
 
-requires-python = ">=3.9"
+requires-python = ">=3.10"
 
 dependencies = [
     "numpy",
     "scipy",
-    "astropy >= 4.3",
+    "astropy",
     "pandas",
     "tqdm",
     "matplotlib",