vidrl_upload: Use Human pool <year> as serum_id for human serum refs

Because the `serum_host` field is unreliable in fauna, seasonal-flu
uses substring matches on the `serum_id` field to separate ferret,
human, and mouse sera.¹

Updating the `serum_id` to be `Human pool <year>` so that it can be
matched in seasonal-flu. This also has the side-effect of setting the
`serum_host` field to "human" within fauna because of the `serum_id`
matching in tdb/upload.py.²

¹ <https://github.com/nextstrain/seasonal-flu/blob/89f6cfd11481b2c51c50d68822c18d46ed56db51/workflow/snakemake_rules/download_from_fauna.smk#L93>
² <https://github.com/nextstrain/fauna/blob/88a607db53d36fc91482cae2009eefddf9477f97/tdb/upload.py#L382-L383>

tdb/vidrl_upload.py

@@ -71,8 +71,10 @@ def parse_human_serum_references(human_serum_data, subtype):
             # Use the first match of the potential fields
             if matches is not None:
                 year = matches.group(2)
-                # Follow a standard pattern where serum_id is `SH {year}`
-                new_serum_id = f"SH {year}"
+                # Follow a standard pattern where serum_id is `Human pool <year>`
+                # Need "human" in serum_id because this is how we match for human sera in seasonal flu
+                # <https://github.com/nextstrain/seasonal-flu/blob/89f6cfd11481b2c51c50d68822c18d46ed56db51/workflow/snakemake_rules/download_from_fauna.smk#L93>
+                new_serum_id = f"Human pool {year}"
                 break
 
         # year is required to know which vaccine reference strain to use,