setup.py: get rid of tests_require

It has the same problems as setup_requires.

setup.py

@@ -616,7 +616,6 @@ def main():
                     'build_tut': build_tut,
                     'test': test},
           ext_modules=exts,
-          tests_require=['numpy > 1.6.1', 'pytest >= 2.6.3'],
           install_requires=['numpy > 1.6.1', 'scipy >= 0.11.0', 'tinyarray'],
           extras_require={'plotting': 'matplotlib >= 1.2'},
           classifiers=[c.strip() for c in classifiers.split('\n')])