try cf-units compatible release notation

Author: bjlittle

setup.cfg

@@ -46,7 +46,7 @@ version = attr: iris.__version__
 include_package_data = True
 install_requires =
     cartopy>=0.20
-    cf-units==3.0.1.post0
+    cf-units~=3.0.1
     cftime==1.5.1.1
     dask[array]>=2
     matplotlib