Install sphinx on CircleCI

.circleci/config.yml

@@ -1,9 +1,9 @@
-version: 2.0
+version: 2
 jobs:
-  build:
+  linux-system-python:
     docker:
         - image: kovidgoyal/kitty-test:latest
     steps:
         - checkout
-        - run: echo "A first hello"
         - run: if grep -Inr '\s$' kitty kitty_tests kittens docs *.py *.asciidoc *.rst .gitattributes .gitignore; then echo Trailing whitespace found, aborting.; exit 1; fi
+        - run: flake8 --count .

.circleci/images/Dockerfile

@@ -2,5 +2,7 @@ FROM ubuntu:latest
 
 RUN apt-get update && apt-get install -y libgl1-mesa-dev libxi-dev libxrandr-dev libxinerama-dev libxcursor-dev libxcb-xkb-dev libdbus-1-dev libxkbcommon-dev libharfbuzz-dev python3 python3-flake8 python3-pip
 
+RUN pip3 install sphinx
+
 LABEL com.circleci.preserve-entrypoint=true
 ENTRYPOINT sleep 2h