Support Python 3.8

[fpoli]

The tests fail on Python 3.8 :

$ nosetests
EF
======================================================================
ERROR: test_export_json (export_test.TestExportJson)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/fpoli/python-astexport/tests/export_test.py", line 16, in test_export_json
    result = json.loads(export_json(test.input))
  File "/home/travis/build/fpoli/python-astexport/astexport/export.py", line 13, in export_json
    export_dict(tree),
  File "/home/travis/build/fpoli/python-astexport/astexport/export.py", line 27, in export_dict
    return DictExportVisitor().visit(tree)
  File "/home/travis/build/fpoli/python-astexport/astexport/export.py", line 36, in visit
    return meth(node)
  File "/home/travis/build/fpoli/python-astexport/astexport/export.py", line 51, in default_visit
    args[field] = meth(getattr(node, field))
  File "/home/travis/build/fpoli/python-astexport/astexport/export.py", line 67, in default_visit_field
    return [self.visit(x) for x in val]
  File "/home/travis/build/fpoli/python-astexport/astexport/export.py", line 67, in <listcomp>
    return [self.visit(x) for x in val]
  File "/home/travis/build/fpoli/python-astexport/astexport/export.py", line 36, in visit
    return meth(node)
  File "/home/travis/build/fpoli/python-astexport/astexport/export.py", line 51, in default_visit
    args[field] = meth(getattr(node, field))
  File "/home/travis/build/fpoli/python-astexport/astexport/export.py", line 65, in default_visit_field
    return self.visit(val)
  File "/home/travis/build/fpoli/python-astexport/astexport/export.py", line 36, in visit
    return meth(node)
  File "/home/travis/build/fpoli/python-astexport/astexport/export.py", line 51, in default_visit
    args[field] = meth(getattr(node, field))
AttributeError: 'Constant' object has no attribute 'kind'
======================================================================
FAIL: test_parse (parse_test.TestParse)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/fpoli/python-astexport/tests/parse_test.py", line 17, in test_parse
    self.assertEqual(
AssertionError: "Modu[99 chars]fset=1)], value=Constant(value=5, kind=None, l[145 chars]=[])" != "Modu[99 chars]fset=0)], value=Constant(value=5, lineno=1, co[98 chars]0)])"
- Module(body=[Assign(targets=[Name(id='x', ctx=Store(), lineno=1, col_offset=0, end_lineno=1, end_col_offset=1)], value=Constant(value=5, kind=None, lineno=1, col_offset=4, end_lineno=1, end_col_offset=5), type_comment=None, lineno=1, col_offset=0, end_lineno=1, end_col_offset=5)], type_ignores=[])
+ Module(body=[Assign(targets=[Name(id='x', ctx=Store(), lineno=1, col_offset=0, end_lineno=1, end_col_offset=0)], value=Constant(value=5, lineno=1, col_offset=4, end_lineno=1, end_col_offset=0), lineno=1, col_offset=0, end_lineno=1, end_col_offset=0)])