add python_requires to setup.cfg

Author: bollwyvl

setup.cfg

@@ -1,6 +1,6 @@
 [metadata]
 name = jupyter-lsp
-description = Multi-Language Server WebSocket proxy for Jupyter notebook or lab server for Python 3.6+.
+description = Multi-Language Server WebSocket proxy for Jupyter Notebook/Lab server
 long_description = file: ./py_src/jupyter_lsp/README.md
 long_description_content_type = text/markdown
 url = https://github.com/krassowski/jupyterlab-lsp
@@ -26,6 +26,7 @@ package_dir =
 packages = find:
 include_package_data = True
 zip_safe = False
+python_requires = >=3.6
 
 install_requires =
     notebook >=4.3.1