Pin yapf version to match current formatting (#131)

Author: siebert

setup.py

@@ -64,7 +64,7 @@
     extras_require={
         ':python_version == "2.7"': ['futures'],
         'test': TEST_REQUIRES,
-        'dev': ['pycodestyle', 'pylint', 'yapf'],
+        'dev': ['pycodestyle', 'pylint', 'yapf==0.23.0'],
     },
     classifiers=[
         'Development Status :: 3 - Alpha',