Discontinue use of tests_require

To achieve compatibility with some of datalad's default CI setups.

setup.cfg

@@ -15,12 +15,15 @@ classifiers =
 python_requires = >= 3.5
 install_requires =
     datalad >= 0.12.0
-test_requires =
-    nose
-    coverage
 packages = find:
 include_package_data = True
 
+[options.extras_require]
+# this matches the name used by -core and what is expected by some CI setups
+devel =
+    nose
+    coverage
+
 [options.packages.find]
 # do not ship the build helpers
 exclude=