Add six to install_requires.

snowdialect.py requires six 1.4.0 or later becaue it uses six.moves.urllib_parse.unquote_plus.

Author: masamitsu-murase

setup.py

@@ -35,6 +35,7 @@
     install_requires=[
         'sqlalchemy<2.0.0,>=1.4.0',
         'snowflake-connector-python<3.0.0',  # Keep in sync with extras dependency
+        'six>=1.4.0',
     ],
     packages=[
         'snowflake.sqlalchemy',