label mistune as python3 only support.

Earlier mistunes that support python2 have a different set of
exports/symbols.

Author: rouilj

doc/installation.txt

@@ -258,7 +258,7 @@ docutils
 markdown, markdown2 or mistune
   To use markdown rendering you need to have the markdown_,
   markdown2_ (2.4.9 known to be broken, 2.3.3 known to work),
-  or mistune_ (v0.8.4 tested) package installed.
+  or mistune_ (v0.8.4 tested; python3 only) package installed.
 
 zstd, brotli
   To have roundup compress the returned data using one of these