Switch to built-in setuptools functionality

Looks like setuptools-markdown is deprecated in favor of a built-in
setuptools functionality. Worse is that setuptools_markdown isn't
compatible with a recent versions of a pypandoc library. Let's just drop
it.

See also man-group/pytest-plugins#87.

Signed-off-by: Peter Lemenkov <[email protected]>

Author: lemenkov

setup.py

@@ -40,19 +40,21 @@
     extras_require['doc']
 )
 
+with open('README.md', encoding="utf-8") as f:
+    long_description = f.read()
 
 setup(
     name='rlp',
     # *IMPORTANT*: Don't manually change the version here. See README for more.
     version='3.0.0',
     description="A package for Recursive Length Prefix encoding and decoding",
-    long_description_markdown_filename='README.md',
+    long_description=long_description,
+    long_description_content_type="text/markdown",
     author="jnnk",
     author_email='[email protected]',
     url='https://github.com/ethereum/pyrlp',
     packages=find_packages(exclude=["tests", "tests.*"]),
     include_package_data=True,
-    setup_requires=['setuptools-markdown'],
     install_requires=[
         "eth-utils>=2.0.0,<3",
     ],