Add python_requires to help pip

When old Python versions are dropped, this will help pip install the right version for people still running those old Python versions.
 
For more info on how this works:
* https://hackernoon.com/phasing-out-python-runtimes-gracefully-956f112f33c4
* pypa/packaging.python.org#450

Author: hugovk

setup.py

@@ -21,6 +21,7 @@
         author_email='[email protected]',
         packages=['twarc',],
         description='Archive tweets from the command line',
+        python_requires='>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*',
         install_requires=dependencies,
         setup_requires=['pytest-runner'],
         tests_require=['pytest'],