Bugfix: drop Python 3.6 (#1556)

* Drop Python 3.6 and test with Python 3.9

* Bugfix: increase `python_requires` to >=3.7

This finalises the drop in support for Python 3.6

Co-authored-by: Sylvain Corlay <sylvain.corlay@gmail.com>

.travis.yml

@@ -7,9 +7,9 @@ language: python
 
 matrix:
     include:
-        - python: 3.6
         - python: 3.7
         - python: 3.8
+        - python: 3.9
     allow_failures:
         - python: nightly
 env:

setup.py

@@ -188,7 +188,7 @@ def get_data_files():
     package_data    = package_data,
     data_files      = get_data_files(),
     cmdclass        = cmdclass,
-    python_requires = '>=3.6',
+    python_requires = '>=3.7',
     author          = 'Jupyter Development Team',
     author_email    = 'jupyter@googlegroups.com',
     url             = 'https://jupyter.org',