Switch to setuptools-scm for versioning

Following other pytest projects like pytest-html,
that use setuptool-scm to manage version, it would
be nice other projects follow the suit and
let setuptools-scm manage version of this project.

pytest_asyncio/__init__.py

@@ -1 +0,0 @@
-__version__ = '0.5.0'

setup.py

@@ -1,29 +1,11 @@
-import codecs
-import os
-import re
 from setuptools import setup, find_packages
 
 with open('README.rst') as f:
     readme = f.read()
 
-
-def read(*parts):
-    here = os.path.abspath(os.path.dirname(__file__))
-    return codecs.open(os.path.join(here, *parts), 'r').read()
-
-
-def find_version(*file_paths):
-    version_file = read(*file_paths)
-    version_match = re.search(r"^__version__ = ['\"]([^'\"]*)['\"]",
-                              version_file, re.M)
-    if version_match:
-        return version_match.group(1)
-
-    raise RuntimeError("Unable to find version string.")
-
 setup(
     name='pytest-asyncio',
-    version=find_version('pytest_asyncio', '__init__.py'),
+    use_scm_version=True,
     packages=find_packages(),
     url='https://github.com/pytest-dev/pytest-asyncio',
     license='Apache 2.0',
@@ -41,6 +23,7 @@ def find_version(*file_paths):
         "Topic :: Software Development :: Testing",
         "Framework :: Pytest",
     ],
+    setup_requires=['setuptools_scm'],
     install_requires=[
         'pytest >= 3.0.2',
     ],