Constrain sphinxcontrib-asyncio to 0.2.*

Fixes this message:

  File "/home/circleci/repo/.tox/py36-docs/lib/python3.6/site-packages/sphinxcontrib/asyncio.py", line 2, in <module>
    from sphinx.domains.python import PyFunction, PyMethod
ImportError: cannot import name 'PyFunction'

Author: carver

setup.py

@@ -54,7 +54,7 @@
         # Sphinx pined to `<1.8.0`: https://github.com/sphinx-doc/sphinx/issues/3494
         "Sphinx>=1.5.5,<1.8.0",
         "sphinx_rtd_theme>=0.1.9",
-        "sphinxcontrib-asyncio>=0.2.0",
+        "sphinxcontrib-asyncio>=0.2.0,<0.3",
         "towncrier>=19.2.0, <20",
     ],
     'dev': [