Fix sphinx-doc#777 (part Ia): Latex output "too deeply nested"

Allow code-blocks at maximal nesting depth of lists/quotes in LaTeX
(which by default is 6), by patching fancyvrb's original Verbatim
dealings. If code-block appears at nesting depth 6 and \@listvii (which
is supposed to configure margins) exists, it will be used, else the
\csname ... \endcsname in fancyvrb's \FV@ListNesting creates a \relax.

This is second commit improving maximal depth for code-blocks in nested
lists or quote blocks. It used to be 4, it is now at 6.

sphinx/texinputs/sphinx.sty

@@ -334,6 +334,8 @@
        }%
   % go around fancyvrb's check of @currenvir (for case of minipage below)
   \renewcommand*{\VerbatimEnvironment}{\gdef\FV@EnvironName{Verbatim}}%
+  % go around fancyvrb's check of current list depth
+  \def\@toodeep {\advance\@listdepth\@ne}%
   % Allow breaks at special characters using \PYG... macros.
   \Sphinxbreaksatspecials
   % The list environment is needed to control perfectly the vertical space.