pythonPackages.markdown: add missing setuptools to propagatedBuildInput

Fixes the following ImportError on application startup:

/nix/store/qh7ndfsar3icmwqbiwcla7pc8x1133vg-python2.7-Markdown-3.1.1/bin/markdown_py README.md > README.html.new
Traceback (most recent call last):
  File "/nix/store/qh7ndfsar3icmwqbiwcla7pc8x1133vg-python2.7-Markdown-3.1.1/bin/.markdown_py-wrapped", line 7, in <module>
    from markdown.__main__ import run
  File "/nix/store/qh7ndfsar3icmwqbiwcla7pc8x1133vg-python2.7-Markdown-3.1.1/lib/python2.7/site-packages/markdown/__init__.py", line 25, in <module>
    from .core import Markdown, markdown, markdownFromFile
  File "/nix/store/qh7ndfsar3icmwqbiwcla7pc8x1133vg-python2.7-Markdown-3.1.1/lib/python2.7/site-packages/markdown/core.py", line 29, in <module>
    import pkg_resources
ImportError: No module named pkg_resources
make: *** [Makefile:53: README.html] Error 1