Netpbmfile

- Read multi-image files.
- Fix reading ASCII formats with trailing comments.
- Fix writing maxval=1, depth=1 binary images.
- Use tifffile.imshow for multi-image arrays if installed.
- Change tupltypes to bytes according to specification (breaking).

- Allow space after token value in PAM.
- Update metadata.

- Add type hints.
- Support reading PF4 RGBA FloatMaps.
- Drop support for Python 3.7 and numpy < 1.19 (NEP29).

- Fix unclosed file warnings.
- Support reading PGX JPEG2000 reference images.

- Disallow comments after last value in PNM headers.

- Drop support for Python 3.6 (NEP 29).
- Support os.PathLike file names.