Install Sphinx upfront in CI

This is a bit of a hack though it fixes CI for now. It's not clear
why letting setuptools install Sphinx as part of the `install_requires`
dependencies doesn't work, however as we're likely going to need
to rework how installs work soon anyway (since python packaging
as a whole is shifting) it doesn't seem worth spending too much
time on fixing it right now.

.circleci/config.yml

@@ -36,6 +36,7 @@ references:
       command: |
         python3 -m venv venv
         venv/bin/pip install -U pip setuptools wheel
+        venv/bin/pip install -U 'Sphinx <7'
         venv/bin/pip install -r all-requirements.txt
   save-dependencies-cache: &save-dependencies-cache
     save_cache:

setup.py

@@ -53,6 +53,8 @@ def run(self):
         'xlwt-future >=0.8, <1',
     ],
     setup_requires=[
+        # Note: Sphinx needs to be manually installed before running `setup.py`
+        # to avoid issues importing `sphinxcontrib.applehelp`.
         'Sphinx >=2, <7',
         'Pygments >=2.0, <3',
     ],