Add type hints

Author: hugovk

src/PIL/Image.py

@@ -3342,7 +3342,7 @@ def _open_core(fp, filename, prefix, formats):
 # Image processing.
 
 
-def alpha_composite(im1, im2):
+def alpha_composite(im1: Image, im2: Image) -> Image:
     """
     Alpha composite im2 over im1.
 
@@ -3357,7 +3357,7 @@ def alpha_composite(im1, im2):
     return im1._new(core.alpha_composite(im1.im, im2.im))
 
 
-def blend(im1, im2, alpha):
+def blend(im1: Image, im2: Image, alpha: float) -> Image:
     """
     Creates a new image by interpolating between two input images, using
     a constant alpha::
@@ -3380,7 +3380,7 @@ def blend(im1, im2, alpha):
     return im1._new(core.blend(im1.im, im2.im, alpha))
 
 
-def composite(image1, image2, mask):
+def composite(image1: Image, image2: Image, mask: Image) -> Image:
     """
     Create composite image by blending images using a transparency mask.