docs: def replace_braces(s, env):
      """Makes tox substitutions to s, with respect to environment env.
  
      Example
      -------
      >>> replace_braces("echo {posargs:{env:USER:} passed no posargs}")
      "echo andy passed no posargs"
  
      Note: first "{env:USER:}" is replaced with os.environ.get("USER", ""),
      the "{posargs:andy}" is replaced with "andy" (since no posargs were
      passed).
  
      """
      def replace(m):
          return _replace_match(m, env)
      for _ in range(DEPTH):
          s = re.sub(r"{[^{}]*}", replace, s)
      return s