text: Title: tabula.io.read_pdf 'columns' argument change type annotation to Iterable[float]
  Body: **Is your feature request related to a problem? Please describe.**
  <!--- A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] -->
  
  The thing is that I tried to give some meaning to the coordinates and wanted to use NamedTuple instead of List, Tuple, etc. In case of the argument "area" it works as it uses more general typing - Iterable[float]. But columns typed as List[float] so the linter gives an error in that case.
  
  **Describe the solution you'd like**
  <!--- A clear and concise description of what you want to happen. -->
  
  I wanted to suggest changing it to Iterable[float] as well if there is no any specific reason using List[float].
  
  **Describe alternatives you've considered**
  <!--- A clear and concise description of any alternative solutions or features you've considered. -->
  
  n/a
  
  **Additional context**
  <!--- Add any other context or screenshots about the feature request here. -->
  
  n/a