Add six to install_requires.

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

Author: masamitsu-murase

setup.cfg

@@ -50,6 +50,7 @@ install_requires =
     sqlalchemy<2.0.0,>=1.4.0
 ; Keep in sync with extras dependency
     snowflake-connector-python<3.0.0
+    six>=1.4.0
 include_package_data = True
 package_dir =
     =src