py-metomi-isodatetime: fix url parsing (#41415)

* py-metomi-isodatetime: fix url parsing

* One-liner

* Add note that checksum doesn't work

var/spack/repos/builtin/packages/py-metomi-isodatetime/package.py

@@ -10,6 +10,7 @@ class PyMetomiIsodatetime(PythonPackage):
     """Python ISO 8601 date time parser and data model/manipulation utilities."""
 
     homepage = "https://github.com/metomi/isodatetime"
+    # NOTE: spack checksum does not yet work for epoch versions
     pypi = "metomi-isodatetime/metomi-isodatetime-1!3.0.0.tar.gz"
 
     maintainers("LydDeb")
@@ -18,3 +19,6 @@ class PyMetomiIsodatetime(PythonPackage):
     version("3.0.0", sha256="2141e8aaa526ea7f7f1cb883e6c8ed83ffdab73269658d84d0624f63a6e1357e")
 
     depends_on("py-setuptools", type="build")
+
+    def url_for_version(self, version):
+        return f"https://files.pythonhosted.org/packages/source/m/metomi-isodatetime/metomi-isodatetime-1!{version}.tar.gz"