Issues with DateTime ISO fields

[JasperCheung]

when i run gql_run and the renderer_dataclasses.py encounters a DateTime field it generates a field:

var_name: datetime = None

when I run a query that returns a datetime object I get the following error:

File "/Users/jasper/Documents/test/test-graph-python-client/env/lib/python3.7/site-packages/dataclasses_json/core.py", line 220, in _decode_generic
    res = _get_type_cons(type_)(xs)
  File "/Users/jasper/Documents/test/test-graph-python-client/env/lib/python3.7/site-packages/dataclasses_json/core.py", line 260, in <genexpr>
    for x in xs)
  File "/Users/jasper/Documents/test/test-graph-python-client/env/lib/python3.7/site-packages/dataclasses_json/core.py", line 165, in _decode_dataclass
    infer_missing)
  File "/Users/jasper/Documents/test/test-graph-python-client/env/lib/python3.7/site-packages/dataclasses_json/core.py", line 179, in _decode_dataclass
    dt = datetime.fromtimestamp(field_value, tz=tz)
TypeError: an integer is required (got type str)

I think this is because in:

gql-next/gql/renderer_dataclasses.py

Lines 153 to 160 in 5c81e58

	if field.type == 'DateTime':
	suffix = '= DATETIME_FIELD'
	field_type = 'datetime'
	if field.nullable:
	suffix = f'= {field.default_value}'
	buffer.write(f'{field.name}: {field_type} {suffix}')

suffix is being overwritten if the field is nullable and DATETIME_FIELD isn't being used.