Increase changelog coverage for widely used packages

[jayfk]

Top 300 packages on pyup.io without a changelog:

• flake8
• pyyaml
• six
• pytz (has no up-to-date changelog https://bugs.launchpad.net/pytz/+bug/1677527, unable to translate changelog releases to actual PyPi releases)
• mock (is now part of Python, changelog seems to be .gitignored: https://github.com/testing-cabal/mock/blob/master/.gitignore#L17)
• factory-boy
• jinja2
• docutils
• sphinx-rtd-theme
• whitenoise
• numpy
• beautifulsoup4
• mccabe
• django-braces
• alabaster
• dj-database-url (has no changelog)
• cffi
• django-coverage-plugin
• newrelic
• pandas
• twine
• pep8-naming
• snowballstemmer (seems to have no changelog)
• certifi (seems there's no changelog available, #Changelog certifi/python-certifi#57)
• django-storages-redux
• enum34 (has no changelog)
• pbr
• docopt (unparseable, project seems to be dead: https://github.com/docopt/docopt)
• wsgiref (no changelog)
• imagesize
• fabric
• ipython-genutils
• simplegeneric
• traitlets
• python-memcached
• pickleshare
• ptyprocess
• django-redis-cache
• bcrypt
• ipaddress
• pyasn1-modules
• requests-mock
• flask-debugtoolbar
• twisted
• pymongo
• configparser
• termcolor
• ujson
• cssmin
• appnope
• webencodings
• unicode-slugify
• reportlab
• tqdm
• unittest2
• python-magic
• olefile
• flake8-blind-except
• flake8-quotes
• pathtools
• fake-factory
• jupyter
• pytest-asyncio
• python-coveralls
• pytest-timeout
• sphinxcontrib-spelling
• unicodecsv
• drf-extensions
• cnx-archive
• pytest-travis-fold
• django-picklefield

[palfrey]

Jinja2 says no changelog. See http://jinja.pocoo.org/docs/2.10/changelog/

[Harmon758]

pbr changelog seems to be missing as well
See #108

[Harmon758]

pandas changelog seems to be missing as well now
See #180