Try resolve docs build failures

doc/doc-requirements.txt

@@ -1,15 +1,14 @@
 # documentation specific extra packages
 myst-parser
+pydata-sphinx-theme
 sphinx-copybutton
 traitlets
 pandas
 ruamel.yaml
-pydata_sphinx_theme
-sphinx>=1.7
 # install BinderHub dependencies. We manually list them here because some
 # dependencies (like pycurl) can't be installed on ReadTheDocs and aren't
 # needed to build the docs.
-kubernetes==9.0.*
+kubernetes
 escapism
 traitlets
 docker

readthedocs.yml

@@ -1,7 +1,24 @@
-name: binderhub
-type: sphinx
-build:
-  image: latest
-requirements_file: doc/doc-requirements.txt
+# Configuration on how ReadTheDocs (RTD) builds our documentation
+# ref: https://docs.readthedocs.io/en/stable/config-file/v2.html
+
+# Required (RTD configuration version)
+version: 2
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+  configuration: doc/conf.py
+
+# Optionally build your docs in additional formats such as PDF and ePub
+formats: []
+
+# Optionally set the version of Python and requirements required to build your docs
 python:
-  version: 3
+  version: 3.7
+  install:
+    # WARNING: This requirements file will be installed without the pip
+    #          --upgrade flag in an existing environment. This means that if a
+    #          package is specified without a lower boundary, we may end up
+    #          accepting the existing version.
+    #
+    #          ref: https://github.com/readthedocs/readthedocs.org/blob/0e3df509e7810e46603be47d268273c596e68455/readthedocs/doc_builder/python_environments.py#L335-L344
+    - requirements: doc/doc-requirements.txt