Use environment markers for custom PyPy requirements

setup.py

@@ -1,11 +1,31 @@
 from os.path import dirname, join
-from setuptools import setup, find_packages
+from pkg_resources import parse_version
+from setuptools import setup, find_packages, __version__ as setuptools_version
 
 
 with open(join(dirname(__file__), 'scrapy/VERSION'), 'rb') as f:
     version = f.read().decode('ascii').strip()
 
 
+def has_environment_marker_platform_impl_support():
+    """Code extracted from 'pytest/setup.py'
+    https://github.com/pytest-dev/pytest/blob/7538680c/setup.py#L31
+
+    The first known release to support environment marker with range operators
+    it is 18.5, see:
+    https://setuptools.readthedocs.io/en/latest/history.html#id235
+    """
+    return parse_version(setuptools_version) >= parse_version('18.5')
+
+
+extras_require = {}
+
+if has_environment_marker_platform_impl_support():
+    extras_require[':platform_python_implementation == "PyPy"'] = [
+        'PyPyDispatcher>=2.1.0',
+    ]
+
+
 setup(
     name='Scrapy',
     version=version,
@@ -53,4 +73,5 @@ setup(
         'PyDispatcher>=2.0.5',
         'service_identity',
     ],
+    extras_require=extras_require,
 )

tox.ini

@@ -57,7 +57,6 @@ commands =
 [testenv:pypy]
 basepython = pypy
 commands =
-    pip install PyPyDispatcher>=2.0.6
     py.test {posargs:scrapy tests}
 
 [testenv:py33]