setup.py: support running in minimal locales

This is moot on most systems but a minimal docker container
with LC_ALL=C would previously fail:

LC_ALL=C LANG=C python3.6 setup.py --long-description

Closes LibraryOfCongress/concordia#69

Author: acdha

setup.py

@@ -8,14 +8,15 @@
 import subprocess
 import sys
 from setuptools import setup
+from codecs import open
 
 if sys.version_info < (2, 7):
     print("Python 2.7 or higher is required")
     sys.exit(1)
 
 description = 'Create and validate BagIt packages'
 
-with open('README.rst') as readme:
+with open("README.rst", encoding="utf-8") as readme:
     long_description = readme.read()
 
 tests_require = ['mock', 'coverage']