Fix pygments style

New theme of docs respects pygments_style.

more info: http://lists.openstack.org/pipermail/openstack-discuss/2020-May/014971.html

Change-Id: I4fbc5fc613e6eb64997935422697477db3ebcc91
diff --git a/doc/source/conf.py b/doc/source/conf.py
index dd7c544..ded713d 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -112,7 +112,7 @@
 show_authors = False
 
 # The name of the Pygments (syntax highlighting) style to use.
-pygments_style = 'sphinx'
+pygments_style = 'native'
 
 # A list of ignored prefixes for module index sorting.
 modindex_common_prefix = ['tempest.']