Port to non-MacOS platforms

[dvdkon]

This PR allows osxphotos to run on non-MacOS platforms (tested on NixOS Linux), both as a library and a CLI tool. This is accomplished mainly by gating specific functionality behind OS checks and providing alternative codepaths where necessary.

This mostly doesn't bring any complexity to the codebase, except for the need to gate new MacOS specific code and the need to deal with Unicode normalisation. MacOS does it in the background, other OSs can theoretically deal with any form of Unicode, but it seems Photos databases aren't internally consistent, so the best solution is to normalise on output.

Two tests are failing, tests/test_cli.py::test_export_touch_files and test_export_touch_files_update. I don't understand why only 31 files should be touched, so I can't fix the test. Other tests are either skipped or run fine.

A port to Linux will allow running tests on GitHub's CI platform, but that's not part of this PR.

[RhetTbull]

Wow -- thank you. This looks like a lot of work. As this is a big PR it will take me some time to review.

[RhetTbull]

@all-contributors please add @dvdkon for code

[allcontributors]

@RhetTbull

I've put up a pull request to add @dvdkon! 🎉

[RhetTbull]

This causes most export tests to fail on macOS. The unicode for macOS must be NFD or anyone who's previously exported unicode filenames with osxphotos will have their exports corrupted (duplicate exports, invalid updates, etc). These file names are chosen because they came from users who had issues with exporting in unicode. For tests to pass on both macOS and linux you'll need separate versions of the test filenames. Can linux worth with NFD filenames? If so, perhaps it would be better to leave everything as NFD on export.

[RhetTbull]

Hi @dvdkon I would love to get this merged but need to get the unicode names in tests fixed. If you're able to add logic to the tests to use the right unicode names depending on platform I can get this merged. Perhaps it might be possible to transform the names from NFC/NFD when importing the test module so you don't need two separate sets of names.

[dvdkon]

Hi, sorry for not getting to this sooner. It would be best to leave Unicode issues to the user and continue as on macOS, but when running on Linux without the changes I've made, many tests fail, since somehow osxphotos is working with both NFC and NFD versions of the same strings. My guess is the Photos DB is not properly normalised, since it doesn't matter on HFS+.

I think your idea of re-normalising the filenames on import based on the platform is the best approach forward. It's going to be strict in requiring that it uses NFD on macOS and NFC elsewhere, but that might be for the better. I'll rebase on master and make the changes soon.

[cclauss]

placate the linter with https://docs.python.org/3/library/contextlib.html

Suggested change

	import locale
	import contextlib
	import locale

[dvdkon]

I got hold of a Mac and can confirm all tests now pass on macOS.

There's still a problem on Linux with two tests checking touched date counts. Apart from that, everything seems OK.

[RhetTbull]

Thanks for the update! I'll run the subset of tests that only run on my machine or require user interaction (timewarp, etc.) this weekend and try to get this merged.