Quick fix for build failure (#57)

The build fails because it can't find sphinxcontrib_trio, which is
is a "dev" dependency. This quick fix is to make it a real dependency.
This might be fixable by doing #61?

setup.py

@@ -39,6 +39,7 @@
         'async_generator',
         'attrs>=18.2',
         'ipaddress',
+        'sphinxcontrib-trio',
         'trio>=0.9,<0.10.0',
         'wsaccel',
         'wsproto>=0.12.0',
@@ -52,7 +53,6 @@
             'pytest-trio>=0.5.0',
             'sphinx',
             'sphinx_rtd_theme',
-            'sphinxcontrib-trio',
             'trustme',
         ],
     },