Remove reassigning variables with new types, pass Mypy

[matt-land]

Changes:

• Minor fixes for "Local variable 'x' might be referenced before assignment" for [width, height] in png.py
• Make function documentation of parameters match type hints where they disagreed or looked out of date
• Fix in gif.py, png.py where optional arguments were assumed truthy (and called) (Mypy arg-type error)
• Function arguments that are optional are changed to be explicit instead of implicit (PEP 484)
  before
  def foo(some_arg: Tree = None):
  after
  def foo(some_Arg: Optional[Tree] = None):
• references that were re-assigned with new types are removed (Mypy "assignment" errors). This was usually from callables to objects, but there are other examples fixed in this pr also. This was also super confusing to contributors. (Issue Possible removal of 'type juggling' of references (mypy related) #59 )

before:

if palette:
    palette = callable(...)  # type: Palette
...
return palette, bitmap

after:

palette_obj = None
if palette:
    palette_obj = callable(...)
...
return palette_obj, bitmap_obj

before:

palette_colors = set()
while something:
    palette.add(<observed color>)
if bitmap:
    palette_colors = list(palette_colors)  # type: List[int]
    bitmap_obj[x, y] = palette_colors.index(pixel)

after:

palette_colors = set()
while something:
    palette.add(<observed color>)
if bitmap:
    bitmap_obj[x, y] = list(palette_colors).index(pixel)

Testing

pipenv shell
pipenv install requirements.txt
mypy .
> Success: no issues found in 16 source files

[matt-land]

@FoamyGuy @kattni Please take a look

[FoamyGuy]

I did a quick pass over the changes this morning and it's looking good to me so far. Thanks for the readability and other improvements @matt-land!

I'll take another look and test it out later tonight.

[FoamyGuy]

This looks good to me. Thanks again @matt-land!

I tested this version successfully on a Feather S2 TFT with a handful of the example scripts and images in this repo.

I've reverted the pre-commit config change for now. I'll make a note in the weeds for the meeting next week to discuss with @kattni and the team how we want to plan to roll out the change to start running mypy and the configuration and versions that it'll use.