Add a git hook for automatic tagging

- When the version field of `pyproject.toml` changes, after the changes have been committed, a tag is created with `git tag` that refers to the new version.
- It's necessary to configure the `git-hooks' directory `git config core.hooksPath .git-hooks`.

.git-hooks/post-commit

@@ -0,0 +1,10 @@
+#! /bin/bash
+version=`git diff HEAD^..HEAD -- "$(git rev-parse --show-toplevel)"/pyproject.toml | grep -m 1 '^\+.*version' | sed -s 's/[^A-Z0-9\.\-]//g'`
+
+if [[ ! $version =~ ^([0-9]+)\.([0-9]+)\.([0-9]+)(\-[A-Z]+\.[0-9]+)?$ ]]; then
+    echo -e "Skip tag: invalid version '$version'"
+    exit 1
+fi
+
+git tag -a "v$version" -m "`git log -1 --format=%s`"
+echo "Created a new tag, $version"