Switch continuous integration to GitHub Actions

This allows freeing up some Travis CI jobs for the main Godot
repository, as job parallelism limits are shared across an
organization.

.github/workflows/ci.yml

@@ -0,0 +1,20 @@
+name: Continuous integration
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Lint and build documentation
+        run: |
+          # Append the `PATH` so we can run `codespell`.
+          export PATH="$HOME/.local/bin:$PATH"
+          pip3 install -r requirements.txt
+          pip3 install codespell
+          bash _tools/format.sh
+          codespell -I _tools/codespell-ignore.txt {about,community,development,getting_started,tutorials}/**/*.rst
+          # Use dummy builder to improve performance.
+          sphinx-build --color -b dummy -d _build/doctrees . _build/html

.travis.yml

@@ -1,34 +0,0 @@
-os: linux
-dist: bionic
-language: python - "3.8"
-
-env:
-  global:
-    - SPHINX_NO_GDSCRIPT=1
-    - SPHINX_NO_SEARCH=1
-    - SPHINX_NO_DESCRIPTIONS=1
-
-matrix:
-  include:
-    - name: Static checks (format.sh) + Sphinx build
-      stage: build
-      os: linux
-      addons:
-        apt:
-          packages:
-            - dos2unix
-            - recode
-
-install:
-  - pip install -r requirements.txt
-  - pip install codespell
-
-script:
-  - bash _tools/format.sh
-
-  # Check for possible typos
-  - codespell -I _tools/codespell-ignore.txt {about,community,development,getting_started,tutorials}/**/*.rst
-
-  # TODO: Add `-W` to turn warnings into errors.
-  # This can only be done once all warnings have been fixed.
-  - sphinx-build --color -b dummy -d _build/doctrees . _build/html