Python 3.7 support in Python Flask generator

Hello,

I've found an error while trying to use the Python Flask generator with Python 3.7 (and 3.8). From the logs I saw a "AttributeError: module 'typing' has no attribute 'GenericMeta'".

The issue is the same described in https://github.com/zalando/connexion/issues/739.

I've changed util.py as for a workaround suggested in that bug, to support both Python 3.6 and 3.7:

elif hasattr(klass, '__origin__'):
    if klass.__origin__ == list or klass.__origin__ == typing.List:
       return _deserialize_list(data, klass.__args__[0])
     if klass.__origin__ == dict:
         return _deserialize_dict(data, klass.__args__[1])

I'm wondering if this (or a better solution) can be ported officially in https://github.com/swagger-api/swagger-codegen-generators/blob/781c5a407c831130285c16eac735d56c0f9edd76/src/main/resources/handlebars/pythonFlaskConnexion/util.mustache

Start by inspecting src/main/resources/handlebars/pythonFlaskConnexion/util.mustache and reproduce the generator failure with Python 3.7. Compare the current typing handling with the reported workaround for Python 3.6 and 3.7. Done means the generated Flask code runs without the GenericMeta error on both versions.