docs/requirements: update to urllib3>=1.23

docs/requirements.txt

@@ -21,7 +21,7 @@ sphinxcontrib-httpdomain==1.5.0
 sphinxcontrib-openapi==0.3.1
 sphinxcontrib-websupport==1.0.1
 typing==3.6.2
-urllib3==1.22
+urllib3>=1.23
 sphinx-tabs==1.1.6
 recommonmark==0.4.0
 sphinxcontrib-spelling