Aligning fstab columns
The type hints you're using for containers are too generic .i.e list, dict etc. You could make them more helpful using the types present in typing module. 

from typing import Dict, List, Union


def format_line(len_dict: Dict[str, str], line: FstabLine) -&gt; str:
def get_longest(lines: List[str]) -&gt; Dict[str, str]:
def parse(content: List[str]) -&gt; List[Union[str, FstabLine]]:


This could also be done for variables like parsed_content as well because Mypy currently throws error for it:


  error: Argument 1 to "append" of "list" has incompatible type
  "FstabLine"; expected "str"


parsed_content: List[Union[str, FstabLine]] = []