docs: autodoc_mock_imports for gssapi

Adds mocking of  and  that cannot be installed on ReadTheDocs
builders.
This fixes the ReadTheDocs building problem.
closes #274

docs/requirements.txt

@@ -4,17 +4,17 @@
 # REANA is free software; you can redistribute it and/or modify it
 # under the terms of the MIT License; see LICENSE file for more details.
 
-"sphinx"
-"sphinx-rtd-theme>=0.1.9"
-"sphinxcontrib-httpdomain>=1.5.0"
-"sphinxcontrib-openapi>=0.3.0,<0.4.0"
-"sphinxcontrib-redoc>=1.5.1"
-"apispec>=0.21.0,<0.40"
-"Flask>=0.11"
-"werkzeug>=0.16.1,<0.17"
-"fs>=2.0"
-"marshmallow>2.13.0,<=2.20.1"
-"reana-commons[kubernetes]>=0.8.0a1,<0.9.0"
-"reana-db>=0.8.0a1,<0.9.0"
-"htcondor==8.9.7"
-"retrying>=1.3.3"
+sphinx>=1.5.1,<1.6
+sphinx-rtd-theme>=0.1.9
+sphinxcontrib-httpdomain>=1.5.0
+sphinxcontrib-openapi>=0.3.0,<0.4.0
+sphinxcontrib-redoc>=1.5.1
+apispec>=0.21.0,<0.40
+Flask>=0.11
+werkzeug>=0.16.1,<0.17
+fs>=2.0
+marshmallow>2.13.0,<=2.20.1
+reana-commons[kubernetes]>=0.8.0a1,<0.9.0
+reana-db>=0.8.0a1,<0.9.0
+htcondor==8.9.7
+retrying>=1.3.3