LaTeX: optional rounded corners for framing of code-blocks

Via a new configuration verbatimradius of 'sphinxsetup', which defaults
to 0pt.

If this dimension is non zero, the LaTeX package pict2e will be loaded
to help construct frames with rounded corners for code-blocks.  And if
the LaTeX package pict2e was not found, a LaTeX warning is issued during
PDF build.

doc/conf.py

@@ -74,6 +74,11 @@ latex_elements = {
              {\footnotesize\raggedright\printindex}
              {\begin{sphinxtheindex}\end{sphinxtheindex}}
 ''',
+    'sphinxsetup': """%
+VerbatimColor={RGB}{242,242,242},%
+VerbatimBorderColor={RGB}{32,32,32},%
+verbatimradius=3pt%
+""",
 }
 latex_show_urls = 'footnote'
 latex_use_xindy = True