Drop support for Python 3.8.

pyproject.toml

@@ -11,7 +11,7 @@ description = "Board and connector definitions for Amaranth HDL"
 authors = [{name = "Amaranth HDL contributors"}]
 license = {file = "LICENSE.txt"}
 
-requires-python = "~=3.8"
+requires-python = "~=3.9"
 dependencies = [
   # this version requirement needs to be synchronized with the one in .github/workflows/main.yml
   "amaranth>=0.4,<0.7",