`copy_ts()` returns non-existent keys

[khaeru]

@Jihoon discovered that copy_ts() returns Keys like:

[<copy ts -2e5ad320a27116a4:>, <copy ts 1df562ba0f86f8c8:>]

which should, instead, be strings like:

["copy ts -2e5ad320a27116a4", "copy ts 1df562ba0f86f8c8"]

Using the former, invalid keys as inputs to further computations leads to a no-op.

This is likely due to the use of genno.core.key.single_key():

message-ix-models/message_ix_models/report/util.py

Lines 191 to 193 in 4378be6

	k1 = rep.add("from_url", f"scenario {_id}", quote(other))
	k2 = rep.add("get_ts", f"ts data {_id}", k1, filters)
	return single_key(rep.add("store_ts", f"copy ts {_id}", "scenario", k2))

Since that's mainly for type checking, we can adjust like:

k_result = f"copy ts {_id}"
rep.add(k_result, "store_ts", "scenario", k2)
return k_result

[Jihoon]

This adjustment from @khaeru worked out well.

rep.add(f"scenario {_id}", "from_url", quote(other))
rep.add(f"ts data {_id}", "get_ts", f"scenario {_id}", filters=filters)
result = f"copy ts {_id}"
rep.add(f"copy ts {_id}", "store_ts", "scenario", f"ts data {_id}")
return result