setuptools-markdown dependency unnecessary

[jluttine]

PyPI supports Markdown formatted README files nowadays: https://packaging.python.org/guides/making-a-pypi-friendly-readme/

Thus, the dependency on setuptools-markdown can be removed. Also from the other eth-* packages.

[jluttine]

Hmm.. I may be wrong, actually. Perhaps the long_description field is required whenever packaging, so it's not PyPI related. Anyway, just for your information.

[carver]

Hm, I'm not sure we've tried this line from the linked docs:

    long_description_content_type='text/markdown'

I'd be happy to get rid of the dependency, but also... it's working and not at the top of my list to test out this change. If someone feels like testing with a dummy pypi deployment to confirm that the content-type change would work, I'm happy to 👍 a PR making the change.

[carver]

This change is implemented in #207