Pinned Sphinx to below 2.0.0 also for py3.5+

Details:

- Sphinx 2.0.0 requires Python >=3.5, so the Sphinx requirement in
  dev-requirements.txt now limits Sphinx to <2.0.0 for Python <3.5.

- Sphinx 2.0.0 fails (e.g. Python 3.7) with an AttributeError in
  the Napoleon extension.
  Issue sphinx-doc/sphinx#6246 has been
  opened. This change mitigates that failure by pinning Sphinx to
  <2.0.0 also for Python >=3.5.

Signed-off-by: Andreas Maier <maiera@de.ibm.com>

dev-requirements.txt

@@ -52,7 +52,11 @@ pytest-cov>=2.4.0,<2.6
 tox>=2.0.0
 
 # Sphinx (no imports, invoked via sphinx-build script, issues on py26):
-Sphinx>=1.7.6; python_version >= '2.7'
+Sphinx>=1.7.6,<2.0.0; python_version >= '2.7'; python_version <	'3.5'  # BSD
+Sphinx>=1.7.6,<2.0.0; python_version >= '3.5'  # BSD
+# TODO: On py3.5+, Sphinx currently fails, see issue
+#       https://github.com/sphinx-doc/sphinx/issues/6246. Therefore, Sphinx has
+#       been pinned to below 2.0.0 also for py3.5+.
 sphinx-git>=10.1.1; python_version >= '2.7'
 GitPython>=2.1.1; python_version >= '2.7'