Make pyproject.toml PEP 621 compliant

- converted tool.poetry to PEP 621 (default behavior for poetry)
- converted setup.py to PEP 621 (recommended by setuptools)
- use pyproject.toml instead of version.txt
- use importlib instead of version.txt
- use PEP 621 dependency format in flatpak build scripts
- added tomli as a dev dependency (used in build scripts)
- removed version.txt (no longer used)
- cleaned up dependencies

- version is still in desktop/setup.py (unclear on how snapcraft uses it)
- poetry lockfiles need to be rebuilt

docs/build.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-VERSION=$(cat ../cli/onionshare_cli/resources/version.txt)
+VERSION=$(cat ../cli/pyproject.toml | python -c 'import tomllib,sys;print(tomllib.load(sys.stdin)["project"]["version"])')
 
 # Supported locales
 LOCALES="en sq bg zh_CN de el ga ja pl ru es tr uk"