Drop setuptools-markdown

Stop using setuptools-markdown which is no longer maintained and
doesn't work anymore. Instead use the long_description_content_type
variable (see [0]) to indicate that the given long_description is in
markdown format.

This fixes:

AttributeError: module 'pypandoc' has no attribute 'convert'

[0] https://dustingram.com/articles/2018/03/16/markdown-descriptions-on-pypi/

Fixes #112

Author: toabctl

setup.py

@@ -19,9 +19,9 @@ def setup_package():
         setup_requires=[
             'six',
             'pyscaffold>=2.5.10,<2.6a0',
-            'setuptools-markdown',
         ] + sphinx,
-        long_description_markdown_filename='README.md',
+        long_description='README.md',
+        long_description_content_type="text/markdown",
         use_pyscaffold=True
     )