Add build with GTK 4

.dockerignore

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+.venv
+__pycache__
+dist
+.mypy_cache
+.github

.github/Dockerfile-gtk4

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+FROM fedora:33
+
+ENV TEST_GTK_VERSION "4.0"
+
+RUN yum install -y gcc gtk4 cairo-devel cairo-gobject-devel gobject-introspection-devel python3-devel python3-pip xorg-x11-server-Xvfb
+RUN pip3 install -U poetry pytest
+
+WORKDIR /work
+
+COPY . /work/
+
+RUN poetry install
+# Override PyGObject with the master version
+RUN poetry run pip install -U https://gitlab.gnome.org/GNOME/pygobject/-/archive/master/pygobject-master.tar.gz
+RUN poetry run xvfb-run pytest --cov=gaphas

.github/workflows/build-gtk4.yml

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+name: build
+on:
+  push:
+
+jobs:
+  build:
+    name: GTK 4
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Build
+        run: docker build -f .github/Dockerfile-gtk4 .