Fix description on PyPI, properly this time

Apparently whatever instructions I was following don't work with
`twine`; this does.  Note that the markdown will only work on new-PyPI,
because old-PyPI doesn't support it; it doesn't seem worthwhile to
rewrite the README to avoid this.

Test plan:
uploaded to https://test.pypi.org/project/slicker/ where it looks pretty

Auditors: csilvers

Author: benjaminjkraft

README.md

@@ -118,6 +118,10 @@ special cases.
 
 ### 0.9.2
 
+- Fix description on PyPI, again
+
+### 0.9.2
+
 - Fix description on PyPI
 
 ### 0.9.1

setup.cfg

@@ -2,10 +2,15 @@
 
 from setuptools import setup
 
+with open("README.md") as f:
+    long_description = f.read()
+
 setup(
     name='slicker',
-    version='0.9.2',
+    version='0.9.3',
     description='A tool for moving python files.',
+    long_description=long_description,
+    long_description_content_type='text/markdown',
     author='Khan Academy',
     author_email='[email protected]',
     url='https://github.com/Khan/slicker',