replace deprecated setuptools-markdown

See:

- msabramo/setuptools-markdown#20
- https://dustingram.com/articles/2018/03/16/markdown-descriptions-on-pypi/

Author: aspiers

setup.cfg

@@ -6,6 +6,7 @@ author_email = git@adamspiers.org
 license = GPL-2+
 home_page = https://github.com/aspiers/git-deps
 long_description = file: README.md, USAGE.md, USE-CASES.md
+long_description_content_type = text/markdown
 classifier =
     Development Status :: 4 - Beta
     Environment :: Console

setup.py

@@ -19,9 +19,7 @@ def setup_package():
         setup_requires=[
             'six',
             'pyscaffold>=2.5.10,<2.6a0',
-            'setuptools-markdown',
         ] + sphinx,
-        long_description_markdown_filename='README.md',
         use_pyscaffold=True
     )