[AG-39] Parse license classifier field from PyPI

[felipead]

We have some Python dependencies where a license is declared as "UNKNOWN" in PyPI. However, a classifier is added with the license.

For example, the package https://pypi.python.org/pypi/backports_abc/json declares the license as "Python Software Foundation License" in:

     "classifiers": [
            "Development Status :: 5 - Production/Stable", 
            "Intended Audience :: Developers", 
            "License :: OSI Approved :: Python Software Foundation License", 
            "Operating System :: OS Independent", 
            "Programming Language :: Python", 
            "Programming Language :: Python :: 2", 
            "Programming Language :: Python :: 3"
        ], 

Also, https://pypi.python.org/pypi/gensim/json has an empty license field, but declares the classifier:

"License :: OSI Approved :: GNU Lesser General Public License v2 or later (LGPLv2+)", 

https://pypi.python.org/pypi/google/json also declares:

"License :: OSI Approved :: BSD License"

https://pypi.python.org/pypi/lockfile/json:

"License :: OSI Approved :: MIT License"

https://pypi.python.org/pypi/msgpack-python/json:

"License :: OSI Approved :: Apache Software License"