Add requires-python >= 3.9 sinve 3.8 is untested

Author: oscarbenjamin

pyproject.toml

@@ -1,7 +1,8 @@
 [project]
 name = "python-flint"
-description = "Bindings for FLINT and Arb"
+description = "Bindings for FLINT"
 version = "0.7.0a4"
+requires-python = ">= 3.9"
 urls = {Homepage = "https://github.com/flintlib/python-flint"}
 authors = [
     {name = "Fredrik Johansson", email = "[email protected]"},
@@ -54,6 +55,8 @@ package = "flint"
 ]
 
 [tool.cibuildwheel]
+# requires-python needs to keep in sync with this and also the list of Python
+# versions the wheels are tested against in CI.
 build = "cp39-* cp310-* cp311-* cp312-*"
 skip = "*-win32 *-manylinux_i686 *-musllinux_*"
 manylinux-x86_64-image = "manylinux2014"