docs: Update sphinxcontrib-openapi

No changes necessary, thankfully, though there is a feature gap here
that we will need 0.7.0 to close [1] :(

[1] sphinx-contrib/openapi#87

Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin

docs/requirements.txt

@@ -2,4 +2,4 @@ sphinx>=1.5
 reno>=2.2
 sphinx_rtd_theme==0.4.3
 jinja2==2.11.1
-sphinxcontrib-openapi==0.5.0
+sphinxcontrib-openapi==0.6.0