Add six to install_requires.

[masamitsu-murase]

install_requires in setup.py should include dependency to six library because snowdialect.py in this library uses six.

I added six>=1.4.0 to install_requires in setup.py because snowdialect.py uses six.moves.urllib_parse.unquote_plus, which was introduced in six 1.4.0.

snowflake-connector-python, which is one of the dependent libraries, v2.7.7 or older depends on six library indirectly, but v2.7.8 does not.
As a result, this issue occurs after snowflake-connector-python v2.7.8 was released recently because six is not installed automatically when a user runs pip install snowflake-sqlalchemy.

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   Add "six>=1.4.0" to install_requires in setup.py.

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[masamitsu-murase]

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[masamitsu-murase]

recheck

[masamitsu-murase]

I rebased this pull request to resolve conflict with 2bde522.