Keep attributes for "bounds" variables

Issue pydata#2921 is about mismatching time units between a time variable and
its "bounds" companion.
However, pydata#2965 does more than fixing pydata#2921, it removes all double
attributes from "bounds" variables which has the undesired side effect
that there is currently no way to save them to netcdf with xarray.
Since the mentioned link is a recommendation and not a hard requirement
for CF compliance, these attributes should be left to the caller to
prepare the dataset variables appropriately if required.
Reduces the amount of surprise that attributes are not written to disk
and fixes pydata#8368.

xarray/conventions.py

@@ -794,24 +794,4 @@ def cf_encoder(variables: T_Variables, attributes: T_Attrs):
 
     new_vars = {k: encode_cf_variable(v, name=k) for k, v in variables.items()}
 
-    # Remove attrs from bounds variables (issue #2921)
-    for var in new_vars.values():
-        bounds = var.attrs["bounds"] if "bounds" in var.attrs else None
-        if bounds and bounds in new_vars:
-            # see http://cfconventions.org/cf-conventions/cf-conventions.html#cell-boundaries
-            for attr in [
-                "units",
-                "standard_name",
-                "axis",
-                "positive",
-                "calendar",
-                "long_name",
-                "leap_month",
-                "leap_year",
-                "month_lengths",
-            ]:
-                if attr in new_vars[bounds].attrs and attr in var.attrs:
-                    if new_vars[bounds].attrs[attr] == var.attrs[attr]:
-                        new_vars[bounds].attrs.pop(attr)
-
     return new_vars, attributes