Pictureshow

Changed

- remove Python 3.6 from supported versions, add Python 3.10
- CLI: if the output filename has no extension specified, `.pdf` will be appended to it

Added

- CI/CD: steps added to GitHub Actions workflow:
- to publish package to PyPI
- to [create GitHub release](https://github.com/softprops/action-gh-release)

Changed

- CLI: pictures do not have to be filesystem paths (can be e.g. URLs)

Changed

- Library API: `PictureShow.save_pdf` and `pictures_to_pdf` now return a named tuple of three values:
- `num_ok`: number of successfully saved pictures
- `errors`: list of items skipped due to error
- `num_pages`: number of pages of the resulting PDF document
- CLI: additional info reported: number of pages of the resulting PDF document

Added

- TESTS: added tests for the command-line app

Added

- Library API: comma is now allowed as delimiter when specifying layout as a string, e.g. `'2,3'` is equivalent to `'2x3'`
- CLI: comma is allowed as delimiter when specifying layout, e.g. `2,3` is equivalent to `2x3`

Changed

- Library API: `pdf_file` is now a required keyword-only argument of the `pictures_to_pdf` function
(all positional arguments are treated as input files)