DOC: fix PDF build for recent LaTeX

Was getting LaTeX error::

    ! LaTeX Error: Too deeply nested.

Fixed using answer at
http://stackoverflow.com/questions/28454217/how-to-avoid-the-too-deeply-nested-error-when-creating-pdfs-with-sphinx

doc/source/conf.py

@@ -252,7 +252,11 @@
 #latex_use_parts = False
 
 # Additional stuff for the LaTeX preamble.
-#latex_preamble = ''
+# Add enumitem to allow deeply-nested lists
+latex_preamble = """
+\usepackage{enumitem}
+\setlistdepth{99}
+"""
 
 # Documents to append as an appendix to all manuals.
 #latex_appendices = []