Make isort use .gitignore properly

By using `--skip / --extend-skip .gitignore`, we actually never read the
.gitignore file. We have to use `--skip-gitignore` instead.

This requires Git in the development environment, so we need to install
Git in our CI runners as well.

.circleci/config.yml

@@ -85,7 +85,7 @@ jobs:
           # https://github.com/freedomofpress/dangerzone/issues/292#issuecomment-1351368122
           command: |
             apt-get update
-            apt-get install -y make python3 python3-poetry --no-install-recommends
+            apt-get install -y git make python3 python3-poetry --no-install-recommends
             poetry install --no-ansi --only lint
       - run:
           name: Run linters to enforce code style

pyproject.toml

@@ -52,7 +52,7 @@ strip-ansi = "*"
 
 [tool.isort]
 profile = "black"
-extend_skip = [".gitignore"]
+skip_gitignore = true
 # This is necessary due to https://github.com/PyCQA/isort/issues/1835
 follow_links = false