Add test for setopt to demonstrate that edit_config retains non-ASCII characters.

setuptools/tests/test_setopt.py

+# coding: utf-8
+
+from __future__ import unicode_literals
+
+import io
+
+import six
+
+from setuptools.command import setopt
+from setuptools.extern.six.moves import configparser
+
+
+class TestEdit:
+    @staticmethod
+    def parse_config(filename):
+        parser = configparser.ConfigParser()
+        with io.open(filename, encoding='utf-8') as reader:
+            (parser.read_file if six.PY3 else parser.readfp)(reader)
+        return parser
+
+    @staticmethod
+    def write_text(file, content):
+        with io.open(file, 'wb') as strm:
+            strm.write(content.encode('utf-8'))
+
+    def test_utf8_encoding_retained(self, tmpdir):
+        """
+        When editing a file, non-ASCII characters encoded in
+        UTF-8 should be retained.
+        """
+        config = tmpdir.join('setup.cfg')
+        self.write_text(config, '[names]\njaraco=йарацо')
+        setopt.edit_config(str(config), dict(names=dict(other='yes')))
+        parser = self.parse_config(str(config))
+        assert parser['names']['jaraco'] == 'йарацо'
+        assert parser['names']['other'] == 'yes'