Fix compatibility with attrs 19.2.0

Author: pquentin

setup.py

@@ -32,7 +32,7 @@
     license='MIT -or- Apache License 2.0',
     packages=find_packages('src'),
     package_dir={'': 'src'},
-    install_requires=['attrs'],
+    install_requires=['attrs>=19.2.0'],
     python_requires='>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*',
     keywords='result',
     classifiers=[

src/outcome/_sync.py

@@ -40,7 +40,7 @@ class Outcome(ABC):
     hashable.
 
     """
-    _unwrapped = attr.ib(default=False, cmp=False, init=False)
+    _unwrapped = attr.ib(default=False, eq=False, init=False)
 
     def _set_unwrapped(self):
         if self._unwrapped: