Require Sphinx < 7 for now

Removal of the setuptools integration from Sphinx breaks our builds. See
sphinx-doc/sphinx#11363 for the change in
Sphinx.

.github/workflows/build.yaml

@@ -32,7 +32,7 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         pip install -r requirements.txt
-        pip install urwid twisted watchdog "jedi >=0.16" babel "sphinx >=1.5"
+        pip install urwid twisted watchdog "jedi >=0.16" babel "sphinx >=1.5,<7"
         pip install pytest pytest-cov numpy
     - name: Build with Python ${{ matrix.python-version }}
       run: |

setup.py

@@ -20,7 +20,7 @@
     from sphinx.setup_command import BuildDoc
 
     # Sphinx 1.5 and newer support Python 3.6
-    using_sphinx = sphinx.__version__ >= "1.5"
+    using_sphinx = sphinx.__version__ >= "1.5" and sphinx.__version__ < "7.0"
 except ImportError:
     using_sphinx = False