Add pre-commit configuration and integration for REUSE.software.

pre-commit.com provides a way to configure pre-commit hooks for multiple
tools, including `reuse lint` (configured here), isort, black, and more.

In addition to `reuse lint`, enable basic whitespace management and YAML
checks. Note that `black` is not usable with this repository because it
cannot parse the template `.py` files.

Use the pre-commit provided GitHub action as part of the CI, for ease of
maintenance.

Author: Flameeyes

.github/workflows/pre-commit.yml

@@ -0,0 +1,24 @@
+# SPDX-FileCopyrightText: Copyright (c) 2019 Anthony Sottile
+#
+# SPDX-License-Identifier: MIT
+
+name: pre-commit
+
+on:
+  pull_request:
+  push:
+    branches: [master]
+
+jobs:
+  pre-commit:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+    - uses: actions/setup-python@v1
+    - name: set PY
+      run: echo "::set-env name=PY::$(python -c 'import hashlib, sys;print(hashlib.sha256(sys.version.encode()+sys.executable.encode()).hexdigest())')"
+    - uses: actions/cache@v1
+      with:
+        path: ~/.cache/pre-commit
+        key: pre-commit|${{ env.PY }}|${{ hashFiles('.pre-commit-config.yaml') }}
+    - uses: pre-commit/[email protected]

.pre-commit-config.yaml

@@ -0,0 +1,15 @@
+# SPDX-FileCopyrightText: 2020 Diego Elio Pettenò
+#
+# SPDX-License-Identifier: Unlicense
+
+repos:
+-   repo: https://github.com/fsfe/reuse-tool
+    rev: latest
+    hooks:
+    - id: reuse
+-   repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v2.3.0
+    hooks:
+    -   id: check-yaml
+    -   id: end-of-file-fixer
+    -   id: trailing-whitespace

readthedocs.yml

@@ -3,4 +3,3 @@
 # SPDX-License-Identifier: Unlicense
 
 requirements_file: requirements.txt
-