Fix doc build error

Doc build keeps failing w/

Recursion error:
    maximum recursion depth exceeded while pickling an object

Sphinx recommends bumping up the recursion limit to
get around this, and it appears to work.

Change-Id: If0a78f9af2e4ddd5fd88c44757642355b3a5bf2e

doc/source/conf.py

@@ -16,6 +16,8 @@ import os
 import sys
 import pbr.version
 
+sys.setrecursionlimit(4000)
+
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.