Fix for "TypeError: 'ellipsis' object is not iterable" error

[Alex314Ru]

Currently Docker version of ethgasstation-backend returns

Traceback (most recent call last):
  File "/opt/ethgasstation/ethgasstation.py", line 8, in <module>
    from egs.main import master_control
  File "/opt/ethgasstation/egs/main.py", line 7, in <module>
    from .egs_ref import *
  File "/opt/ethgasstation/egs/egs_ref.py", line 15, in <module>
    from hexbytes import HexBytes
  File "/usr/local/lib/python3.5/dist-packages/hexbytes/__init__.py", line 1, in <module>
    from .main import HexBytes  # noqa: F401
  File "/usr/local/lib/python3.5/dist-packages/hexbytes/main.py", line 1, in <module>
    from eth_utils import (
  File "/usr/local/lib/python3.5/dist-packages/eth_utils/__init__.py", line 24, in <module>
    from .applicators import (  # noqa: F401
  File "/usr/local/lib/python3.5/dist-packages/eth_utils/applicators.py", line 29, in <module>
    def combine_argument_formatters(*formatters: List[Callable[..., Any]]) -> Formatters:
  File "/usr/lib/python3.5/typing.py", line 1025, in __getitem__
    tvars = _type_vars(params)
  File "/usr/lib/python3.5/typing.py", line 284, in _type_vars
    _get_type_vars(types, tvars)
  File "/usr/lib/python3.5/typing.py", line 279, in _get_type_vars
    t._get_type_vars(tvars)
  File "/usr/lib/python3.5/typing.py", line 786, in _get_type_vars
    _get_type_vars(self.__args__, tvars)
  File "/usr/lib/python3.5/typing.py", line 277, in _get_type_vars
    for t in types:
TypeError: 'ellipsis' object is not iterable

when you try to run built container.

It's the issue of Python 3.5.2 which is used in Ubuntu Xenial by default.
So with this PR we will use Python 3.6 to run the app.