Set python_requires (#29)

Author: rupert

setup.cfg

@@ -8,15 +8,14 @@ url = https://github.com/rupert/pyls-black
 long_description = file: README.md
 long_description_content_type = text/markdown
 classifiers =
-    Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
+    Programming Language :: Python
     License :: OSI Approved :: MIT License
     Operating System :: OS Independent
 
 [options]
 packages = find:
 install_requires = python-language-server; black>=19.3b0; toml
+python_requires = >= 3.6
 
 [options.entry_points]
 pyls = pyls_black = pyls_black.plugin