Pin Python docs build dependencies

To avoid an incompatibility between Breathe 4.6.0 and Sphinx 1.6.x.

.travis.yml

@@ -25,7 +25,7 @@ install:
   - wget https://raw.githubusercontent.com/WrinklyNinja/ci-scripts/1.4.0/install_boost.py
   - python install_boost.py --directory ~ --boost-version 1.64.0 -a 64 -t gcc-6 atomic chrono date_time filesystem iostreams locale log regex system thread
   # Install packages for generating documentation
-  - pip install --user sphinx breathe sphinx_rtd_theme
+  - pip install --user -r docs/requirements.txt
   # Add sphinx-build to PATH
   - export PATH="$HOME/.local/bin:$PATH"
   # Don't install Doxygen from the repositories, it also gets the LaTeX packages

appveyor.yml

@@ -24,7 +24,7 @@ environment:
 
 install:
   - choco install -y doxygen.portable
-  - python -m pip install sphinx breathe sphinx_rtd_theme
+  - python -m pip install -r docs/requirements.txt
   - ps: (New-Object Net.WebClient).DownloadFile('https://raw.githubusercontent.com/WrinklyNinja/ci-scripts/1.1.0/install_boost.py', "$env:APPVEYOR_BUILD_FOLDER\install_boost.py")
   - ps: (New-Object Net.WebClient).DownloadFile('https://raw.githubusercontent.com/WrinklyNinja/ci-scripts/1.2.0/delete_old_bintray_versions.py', "$env:APPVEYOR_BUILD_FOLDER\delete_old_bintray_versions.py")
   - ps: if ($env:PLATFORM -eq "Win32") { $env:ADDRESS_MODEL = '32' } else { $env:ADDRESS_MODEL = '64' }

docs/requirements.txt

@@ -0,0 +1,3 @@
+sphinx==1.5.6
+sphinx_rtd_theme==0.2.4
+breathe==4.6.0