clean up setup.py and use setuptools_scm

setup.py

@@ -2,7 +2,7 @@ from setuptools import setup
 
 setup(
     name="pytest-xdist",
-    version='1.13.dev1',
+    use_scm_version={'write_to': 'xdist/_version.py'},
     description='py.test xdist plugin for distributed testing and loop-on-failing modes',
     long_description=open('README.txt').read(),
     license='MIT',
@@ -20,6 +20,7 @@ setup(
     },
     zip_safe=False,
     install_requires=['execnet>=1.1', 'pytest>=2.4.2', 'py>=1.4.22'],
+    setup_requires=['setuptools_scm'],
     classifiers=[
         'Development Status :: 5 - Production/Stable',
         'Intended Audience :: Developers',

xdist/__init__.py

@@ -1,2 +1 @@
-#
-__version__ = '1.13.dev1'
+from xdist._version import version as __version__