shtab v1.6.2 beta

• 🔨 fix zsh completion not allowing single-letter stacking (#137)
• 🔨 fix tcsh completion for --double-hyphen-only args (#112)
• 💅 tidy code with f-strings (#135)
• 🛠️ misc build/test framework updates (#139)
  • fix docs build (NiklasRosenstein/pydoc-markdown#291)
  • migrate setup.{cfg,py} => pyproject.toml
  • minify *.whl package data
  • update pre-commit tests
  • tidy build/test requirements