Add pre-commit and black into dev-deps

Author: vtitor

setup.py

@@ -13,6 +13,7 @@
     url="https://github.com/pytest-dev/pytest-twisted",
     py_modules=["pytest_twisted"],
     install_requires=["greenlet", "pytest>=2.3", "decorator"],
+    extras_require={"dev": ["pre-commit", "black"]},
     classifiers=[
         "Development Status :: 5 - Production/Stable",
         "Intended Audience :: Developers",