Relabel fix

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Broken expression:

In [1]: ds = bz.Data(resource('postgresql://localhost/bz::ds', schema='vnd'))

In [2]: ds.dshape
Out[2]: 
dshape("""var * {
  asof_date: ?datetime,
  value: ?float64,
  symbol: ?string,
  knowledge_date: ?datetime,
  sid: ?int64
  }""")

In [3]: ds[['value', 'sid']].relabel(value='test')
Out[3]: ---------------------------------------------------------------------------
ProgrammingError                          Traceback (most recent call last)
    ...
ProgrammingError: (psycopg2.ProgrammingError) subquery in FROM must have an alias
LINE 2: FROM (SELECT vnd.ds.value AS value, vnd.ds.sid AS sid 
             ^
HINT:  For example, FROM (SELECT ...) [AS] foo.
 [SQL: 'SELECT value AS test, sid \nFROM (SELECT vnd.ds.value AS value, vnd.ds.sid AS sid \nFROM vnd.ds) \n LIMIT %(param_1)s'] [parameters: {'param_1': 11}]