link_tile returns a new uuid as a bytestring

When you register a Higlass file via `/api/v/link_tile`, the uuid is a byte string instead of an encoded string. In Python 3, it turns the uuid into an encoded string `"b''"`, including the `b''`

I send this payload
```python
{
'coordSystem': 'GRCh38',
'filepath': '/.chrom.sizes',
'filetype': 'chromsizes-tsv',
'datatype': 'chromsizes'
}
```

```python
res = requests.post(
higlass_server + '/api/v1/link_tile/',
data=json.dumps(payload),
auth=authentication,
headers=headers
)
```

The server returns
`{'uuid': "b'Cr1CSDnZSEqMW-Q2lM01Mw'"}`

The uuid should be encoded to match the JSON response.