Added type hints to FontFile and subclasses

[radarhere]

Helps #2625.

Adds type hints to FontFile, BdfFontFile and PcfFontFile.

In doing so,

• I found that

  Pillow/src/PIL/FontFile.py

  Line 68 in 1e8bad8

  return ""

  
  is the only time that FontFile.compile() returns something other than None. I've changed that to be consistent.
• I changed the following error

>>> from PIL import FontFile
>>> font = FontFile.FontFile()
>>> font.save("out")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "PIL/FontFile.py", line 97, in save
    self.bitmap.save(os.path.splitext(filename)[0] + ".pbm", "PNG")
AttributeError: 'NoneType' object has no attribute 'save'

to "ValueError: No bitmap created" instead.

[hugovk]

Great to find and fix some bugs along the way!

[nulano]

I don't see why this one is needed?

Checking with typing.reveal_type, I see that it can be inferred if you add the following at the top of the file:

BYTES_PER_ROW: list[Callable[[int], int]] = [ ...

[radarhere]

Without it, python3 -m mypy --strict src/PIL/PcfFontFile.py gives

src/PIL/PcfFontFile.py:223: error: Call to untyped function (unknown) in typed context  [no-untyped-call]
                    Image.frombytes("1", (xsize, ysize), data[b:e], "raw", mode, pad(xsize))
                                                                                 ^~~~~~~~~~
Found 1 error in 1 file (checked 1 source file)

[radarhere]

Your suggestion also works however, so I've pushed a commit.

[nulano]

Ah, I was missing the --strict.