fix docs build

Author: dimbleby

docs/conf.py

@@ -332,7 +332,6 @@ def autodoc_skip_member(app, what, name, obj, skip, options):
 
 # http://sphinx-doc.org/ext/intersphinx.html
 intersphinx_mapping = {
-   'py3': ('https://docs.python.org/3', None),
    'python': ('https://docs.python.org/3', None),
    'rtd': ('https://docs.readthedocs.io/en/latest/', None),
    'txaio': ('https://txaio.readthedocs.io/en/latest/', None),

requirements-dev.txt

@@ -24,7 +24,7 @@ pytest>=3.4.2
 pyyaml>=4.2b4
 qualname
 sphinx-autoapi>=1.7.0
-sphinx>=1.7.1
+sphinx>=1.7.1,<8
 sphinx_rtd_theme>=0.1.9
 sphinxcontrib-images>=0.9.1
 tox-gh-actions>=2.2.0