BLD: fix compilation issues in sphinx-2.0

Specify explicit path to the html theme.

Author: pavoljuhas

doc/manual/source/conf.py

@@ -20,6 +20,7 @@
 import os
 import sys
 import time
+import importlib
 
 sys.path.insert(0, os.path.abspath('../../..'))
 
@@ -126,6 +127,8 @@
 # a list of builtin themes.
 #
 html_theme = 'sphinx_py3doc_enhanced_theme'
+tmod = importlib.import_module(html_theme)
+html_theme_path = [tmod.get_html_theme_path()]
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the