Preserve encoding with edit_config

PR pypa#1180 and 24be5ab changed the way we read setup.cfg, but when
editing setup.cfg we might still lose the encoding hint on the file,
which can cause failures in subsequent steps.
This change detectes the encoding like in `_parse_config_files` and
writes an encoding hint header if the file encoding was detected.
Fixes jaraco/configparser#37.

setuptools/command/setopt.py

@@ -2,8 +2,11 @@
 from distutils import log
 from distutils.errors import DistutilsOptionError
 import distutils
+import io
 import os
+from ..unicode_utils import detect_encoding
 
+from setuptools.extern import six
 from setuptools.extern.six.moves import configparser
 
 from setuptools import Command
@@ -40,7 +43,14 @@ def edit_config(filename, settings, dry_run=False):
     """
     log.debug("Reading configuration from %s", filename)
     opts = configparser.RawConfigParser()
-    opts.read([filename])
+    try:
+        with io.open(filename, 'rb') as fp:
+            encoding = detect_encoding(fp)
+            log.debug("Reading %s [%s]" % (filename, encoding or 'locale'))
+            reader = io.TextIOWrapper(fp, encoding=encoding)
+            (opts.read_file if six.PY3 else opts.readfp)(reader)
+    except IOError:
+        encoding = 'locale'
     for section, options in settings.items():
         if options is None:
             log.info("Deleting section [%s] from %s", section, filename)
@@ -70,6 +80,8 @@ def edit_config(filename, settings, dry_run=False):
     log.info("Writing %s", filename)
     if not dry_run:
         with open(filename, 'w') as f:
+            if encoding != 'locale':
+                f.write("# coding: %s\n" % encoding)
             opts.write(f)