Try to fix rtd build

python/requirements_dev.txt

@@ -29,5 +29,4 @@ tox~=3.1.2
 mock~=2.0.0
 Sphinx~=1.7.6
 sphinx_rtd_theme~=0.4.0
-nbsphinx~=0.3.3
 pandoc~=1.0.2

python/requirements_rtd.txt

@@ -0,0 +1,22 @@
+# Copyright (c) 2016-2018, Blue Brain Project
+#                          Raphael Dumusc <raphael.dumusc@epfl.ch>
+#                          Daniel Nachbaur <daniel.nachbaur@epfl.ch>
+#                          Cyrille Favreau <cyrille.favreau@epfl.ch>
+#
+# This file is part of Brayns <https://github.com/BlueBrain/Brayns>
+#
+# This library is free software; you can redistribute it and/or modify it under
+# the terms of the GNU Lesser General Public License version 3.0 as published
+# by the Free Software Foundation.
+#
+# This library is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+# FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this library; if not, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+# All rights reserved. Do not distribute without further notice.
+
+nbsphinx~=0.3.3