Fix DeprecationWarning on python 3.7 for python 3.8

[juanmaneo]

• got this with version 3.7.3 of python so fixed it
• add python 3.7 to travis
• ensure the retro compatibility with pypy and python 2.7
• bump version to 3.0.8

[juanmaneo]

@corydolphin You are welcome. All thanks should be to you for taking the time to maintain your very useful python/flask package.