Devops: Add pre-commit hook that validates optional dependencies

The `dev/validate_optional_dependencies.py` is added. It validates that
the `all_plugins` extras specifies exactly the same dependency
requirements that all other extras combined declare as well, except for
the `docs`, `pre-commit`, and `tests` extras, which are only used for
development.

This is to ensure that the `all_plugins` extras provides the exact same
dependencies as all the plugin specific extras combined. The script is
called through a pre-commit hook.

.pre-commit-config.yaml

@@ -31,3 +31,11 @@ repos:
     args: [--autofix]
   - id: pretty-format-yaml
     args: [--autofix]
+
+- repo: local
+  hooks:
+  - id: optional-dependencies
+    name: validate optional dependencies
+    entry: python ./dev/validate_optional_dependencies.py
+    language: system
+    files: pyproject.toml|

dev/validate_optional_dependencies.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python
+"""Script to validate the optional dependencies in the `pyproject.toml`."""
+
+
+def main():
+    """Validate the optional dependencies."""
+    import pathlib
+    import sys
+
+    import tomllib
+
+    filepath_pyproject_toml = pathlib.Path(__file__).parent.parent / 'pyproject.toml'
+
+    with filepath_pyproject_toml.open('rb') as handle:
+        pyproject = tomllib.load(handle)
+
+    exclude = ['all_plugins', 'docs', 'pre-commit', 'tests']
+    dependencies_all_plugins = pyproject['project']['optional-dependencies']['all_plugins']
+    dependencies_separate = []
+
+    for key, dependencies in pyproject['project']['optional-dependencies'].items():
+        if key in exclude:
+            continue
+        dependencies_separate.extend(dependencies)
+
+    missing_all_plugins = set(dependencies_separate).difference(set(dependencies_all_plugins))
+    excess_all_plugins = set(dependencies_all_plugins).difference(set(dependencies_separate))
+
+    if missing_all_plugins:
+        print(
+            'ERROR: the `all_plugins` extras are inconsistent. The following plugin dependencies are missing: '
+            f'{", ".join(missing_all_plugins)}',
+            file=sys.stderr,
+        )
+        sys.exit(1)
+
+    if excess_all_plugins:
+        print(
+            'ERROR: the `all_plugins` extras are inconsistent. The following dependencies are not declared by any '
+            f'plugin extras: {", ".join(excess_all_plugins)}',
+            file=sys.stderr,
+        )
+        sys.exit(1)
+
+
+if __name__ == '__main__':
+    main()