TST: doctest in ibis.bigquery.udf.api is failing for python 3.8

[xmnlab]

A doctest in ibis.bigquery.udf.api is failing for python 3.8

============================================= FAILURES =============================================
_______________________________ [doctest] ibis.bigquery.udf.api.udf ________________________________
070     `the BigQuery documentation
071     <https://cloud.google.com/bigquery/docs/reference/standard-sql/user-defined-functions#sql-type-encodings-in-javascript>`_.
072 
073     Examples
074     --------
075     >>> from ibis.bigquery import udf
076     >>> import ibis.expr.datatypes as dt
077     >>> @udf(input_type=[dt.double], output_type=dt.double)
UNEXPECTED EXCEPTION: NotImplementedError("'visit_Constant' nodes not yet implemented")
Traceback (most recent call last):

  File "/home/xmn/miniconda3/envs/ibis-py38/lib/python3.8/doctest.py", line 1329, in __run
    exec(compile(example.source, filename, "single",

  File "<doctest ibis.bigquery.udf.api.udf[3]>", line 2, in <module>

  File "/home/xmn/dev/quansight/ibis-project/ibis/ibis/bigquery/udf/api.py", line 215, in wrapper
    source = PythonToJavaScriptTranslator(f).compile()

  File "/home/xmn/dev/quansight/ibis-project/ibis/ibis/bigquery/udf/core.py", line 133, in compile
    return self.visit(self.ast)

  File "/home/xmn/dev/quansight/ibis-project/ibis/ibis/bigquery/udf/core.py", line 146, in visit
    result = method(node)

  File "/home/xmn/dev/quansight/ibis-project/ibis/ibis/bigquery/udf/core.py", line 413, in visit_Module
    return '\n\n'.join(map(self.visit, node.body))

  File "/home/xmn/dev/quansight/ibis-project/ibis/ibis/bigquery/udf/core.py", line 146, in visit
    result = method(node)

  File "/home/xmn/dev/quansight/ibis-project/ibis/ibis/bigquery/udf/core.py", line 211, in visit_FunctionDef
    body = indent(map(self.visit, node.body))

  File "/home/xmn/dev/quansight/ibis-project/ibis/ibis/bigquery/udf/core.py", line 49, in indent
    text = '\n'.join(lines)

  File "/home/xmn/dev/quansight/ibis-project/ibis/ibis/bigquery/udf/core.py", line 146, in visit
    result = method(node)

  File "/home/xmn/dev/quansight/ibis-project/ibis/ibis/bigquery/udf/core.py", line 63, in wrapper
    return f(*args, **kwargs) + ';'

  File "/home/xmn/dev/quansight/ibis-project/ibis/ibis/bigquery/udf/core.py", line 232, in visit_Return
    return 'return {}'.format(self.visit(node.value))

  File "/home/xmn/dev/quansight/ibis-project/ibis/ibis/bigquery/udf/core.py", line 146, in visit
    result = method(node)

  File "/home/xmn/dev/quansight/ibis-project/ibis/ibis/bigquery/udf/core.py", line 273, in visit_BinOp
    self.visit(left), self.visit(op), self.visit(right)

  File "/home/xmn/dev/quansight/ibis-project/ibis/ibis/bigquery/udf/core.py", line 141, in visit
    raise NotImplementedError(

NotImplementedError: 'visit_Constant' nodes not yet implemented

[xmnlab]

https://github.com/ibis-project/ibis/issues/2085 proposes to skip this doctest

[tswast]

I think this was actually a regression in Ibis 1.4.0, as I get this same NotImplementedError: 'visit_Constant' nodes not yet implemented when I run the code from the ibis bigquery tutorial.

@ibis.bigquery.udf(['double'], 'double')
def example_udf(value):
    return value + 1.0

test_column = ibis.literal(1, type='double')
expression = example_udf(test_column)

print(conn.execute(expression)

[tswast]

As we are splitting BigQuery to it's own repository (ibis-project/ibis#2665), I'll track the fix for this here: #6