Add types for #56 Missing type annotations

[matt-land]

#56 This is not yet mypy passing, but it is intended for merge as is. It will likely need more work after #59. This is about as good as it gets for now. The remaining issues are due to type juggling (reassigning references from one type to another):

% mypy .
adafruit_imageload/gif.py:54: error: "None" not callable
adafruit_imageload/pnm/ppm_binary.py:53: error: Unsupported target for indexed assignment ("Callable[[int], Palette]")
adafruit_imageload/pnm/ppm_binary.py:63: error: Unsupported target for indexed assignment ("Callable[[int, int, int], Bitmap]")
adafruit_imageload/pnm/ppm_binary.py:63: error: "Set[Tuple[int, int, int]]" has no attribute "index"
adafruit_imageload/pnm/ppm_binary.py:68: error: Incompatible return value type (got "Tuple[Optional[Callable[[int, int, int], Bitmap]], Optional[Callable[[int], Palette]]]", expected "Tuple[Optional[Bitmap], Optional[Palette]]")
adafruit_imageload/pnm/ppm_ascii.py:61: error: Unsupported target for indexed assignment ("Callable[[int], Palette]")
adafruit_imageload/pnm/ppm_ascii.py:69: error: Unsupported target for indexed assignment ("Callable[[int, int, int], Bitmap]")
adafruit_imageload/pnm/ppm_ascii.py:69: error: "Set[bytes]" has no attribute "index"
adafruit_imageload/pnm/ppm_ascii.py:71: error: Incompatible return value type (got "Tuple[Optional[Callable[[int, int, int], Bitmap]], Optional[Callable[[int], Palette]]]", expected "Tuple[Optional[Bitmap], Optional[Palette]]")
adafruit_imageload/pnm/pgm/binary.py:52: error: Unsupported target for indexed assignment ("Callable[[int, int, int], Bitmap]")
adafruit_imageload/pnm/pgm/binary.py:52: error: "Set[int]" has no attribute "index"
adafruit_imageload/pnm/pgm/binary.py:53: error: Incompatible return value type (got "Tuple[Optional[Callable[[int, int, int], Bitmap]], Optional[Callable[[int], Palette]]]", expected "Tuple[Optional[Bitmap], Optional[Palette]]")
adafruit_imageload/pnm/pgm/ascii.py:53: error: Incompatible types in assignment (expression has type "List[int]", variable has type "Set[int]")
adafruit_imageload/pnm/pgm/ascii.py:64: error: Unsupported target for indexed assignment ("Callable[[int, int, int], Bitmap]")
adafruit_imageload/pnm/pgm/ascii.py:64: error: "Set[int]" has no attribute "index"
adafruit_imageload/pnm/pgm/ascii.py:65: error: Incompatible return value type (got "Tuple[Optional[Callable[[int, int, int], Bitmap]], Optional[Callable[[int], Palette]]]", expected "Tuple[Optional[Bitmap], Optional[Palette]]")
adafruit_imageload/bmp/indexed.py:30: error: Incompatible types in assignment (expression has type "None", variable has type "Callable[[Bitmap, BinaryIO, int, int, bool, bool, bool], None]")
adafruit_imageload/bmp/indexed.py:66: error: Unsupported target for indexed assignment ("Callable[[int], Palette]")
adafruit_imageload/bmp/indexed.py:103: error: Argument 1 to "readinto" has incompatible type "Callable[[int, int, int], Bitmap]"; expected "Bitmap"
adafruit_imageload/bmp/indexed.py:126: error: Argument "bitmap" to "decode_rle" has incompatible type "Callable[[int, int, int], Bitmap]"; expected "Bitmap"
adafruit_imageload/bmp/indexed.py:133: error: Incompatible return value type (got "Tuple[Optional[Callable[[int, int, int], Bitmap]], Optional[Callable[[int], Palette]]]", expected "Tuple[Bitmap, Optional[Palette]]")
adafruit_imageload/pnm/__init__.py:95: error: "None" not callable
adafruit_imageload/pnm/__init__.py:98: error: Unsupported target for indexed assignment ("Callable[[int], Palette]")
adafruit_imageload/pnm/__init__.py:106: error: Argument "bitmap" to "load" has incompatible type "Optional[Callable[[int, int, int], Bitmap]]"; expected "Bitmap"
adafruit_imageload/pnm/__init__.py:107: error: Argument "palette" to "load" has incompatible type "Optional[Callable[[int], Palette]]"; expected "Optional[Palette]"
adafruit_imageload/pnm/__init__.py:116: error: Argument "bitmap" to "load" has incompatible type "Optional[Callable[[int, int, int], Bitmap]]"; expected "Bitmap"
adafruit_imageload/pnm/__init__.py:117: error: Argument "palette" to "load" has incompatible type "Optional[Callable[[int], Palette]]"; expected "Optional[Palette]"
adafruit_imageload/__init__.py:78: error: Incompatible return value type (got "Tuple[Optional[Bitmap], Optional[Palette]]", expected "Tuple[Bitmap, Optional[Palette]]")
adafruit_imageload/__init__.py:82: error: Argument "bitmap" to "load" has incompatible type "Optional[Callable[[int, int, int], Bitmap]]"; expected "Callable[[int, int, int], Bitmap]"
Found 29 errors in 8 files (checked 15 source files)

[kattni]

Thanks for all the work that went into this! Couple of simple change suggestions below.

I'll let one of the other folks check this out for type hint content.

[FoamyGuy]

There was a lot to this one, thank you so much for tackling this @matt-land

Types all look good to me and I tested all examples successfully on PyPortal 7.3.0-beta.0