setup: relax dependency on amaranth.

Author: whitequark

setup.py

@@ -21,7 +21,7 @@ def local_scheme(version):
     license="BSD",
     setup_requires=["wheel", "setuptools", "setuptools_scm"],
     install_requires=[
-        "amaranth>=0.2,<0.4",
+        "amaranth>=0.2,<0.5",
         "importlib_metadata; python_version<'3.8'",
     ],
     packages=find_packages(),