Force pygments upgrade

Author: nthiery

setup.py

@@ -31,7 +31,7 @@
     keywords = "SageMath tutorials",
     packages = find_packages(),
     setup_requires   = ['sage-package'],
-    install_requires = ['sage-package', 'sphinx', 'rst2ipynb','nbsphinx','PyYAML', 'sphinxcontrib-jupyter'],
+    install_requires = ['sage-package', 'sphinx', 'pygments>=2.4.1', 'rst2ipynb','nbsphinx','PyYAML', 'sphinxcontrib-jupyter'],
     #dependency_links=['http://github.com/nthiery/rst2ipynb/tarball/master#egg=dev'],
     entry_points = {
         "distutils.commands": [