Remove obsolete, broken pyandoc usage

Pyandoc is no longer supported, and the final version of it on PyPI is
incompatible with how our setup.py was using it. This commit removes
pyandoc entirely and lets Python's setuptools handle the README
conversion from Markdown as needed.

setup.py

@@ -16,25 +16,11 @@
 
 from setuptools import setup
 
-LONG_DESCRIPTION = None
 README_MARKDOWN = None
 
 with open('README.md') as markdown_source:
     README_MARKDOWN = markdown_source.read()
 
-try:
-    import pandoc
-    pandoc.core.PANDOC_PATH = 'pandoc'
-    # Converts the README.md file to ReST, since PyPI uses ReST for formatting,
-    # This allows to have one canonical README file, being the README.md
-    doc = pandoc.Document()
-    doc.markdown = README_MARKDOWN
-    LONG_DESCRIPTION = doc.rst
-except ImportError:
-    # If pandoc isn't installed, e.g. when downloading from pip,
-    # just use the regular README.
-    LONG_DESCRIPTION = README_MARKDOWN
-
 def load_requirements(*requirements_paths):
     """
     Load all requirements from the specified requirements files.
@@ -63,15 +49,15 @@ def is_requirement(line):
     version='0.2.1',
     description='Proxy around MongoDB connection that automatically handles AutoReconnect exceptions.',
     author='Gustav Arngarden',
-    long_description=LONG_DESCRIPTION,
+    long_description=README_MARKDOWN,
+    long_description_content_type='text/markdown',
     classifiers=[
         'License :: OSI Approved :: Apache Software License',
         'Programming Language :: Python',
         'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.8',
     ],
-    setup_requires=['pyandoc'],
     install_requires=load_requirements('requirements/base.in'),
     url="https://github.com/arngarden/MongoDBProxy"
 )