ci: attempt fixing sphinx install

appveyor.yml

@@ -5,7 +5,6 @@ platform:
 
 version: 1.0.{build}
 install:
-- py -m pip install sphinx_rtd_theme
 - ps: Invoke-WebRequest 'https://github.com/embree/embree/releases/download/v3.12.1/embree-3.12.1.x64.vc14.windows.zip' -OutFile 'embree64.zip'
 - ps: 7z x embree64.zip -oc:\
 - ps: Invoke-WebRequest 'https://github.com/oneapi-src/oneTBB/releases/download/v2020.2/tbb-2020.2-win.zip' -OutFile 'tbb.zip'

build-appveyor.ps1

@@ -5,6 +5,15 @@ $env:Path += ";C:\cygwin64\bin"
 # For sha256sum
 $env:Path += ";C:\Program Files\Git\usr\bin"
 
+# Create and activate a Python virtual environment, and install sphinx
+py -m venv ericwtools-env
+Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser
+ericwtools-env\Scripts\Activate.ps1
+python.exe -m pip install sphinx_rtd_theme
+
+# Confirm Sphinx is installed
+get-command sphinx-build
+
 mkdir cmakebuild
 
 cd cmakebuild