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

desktop/setup-freeze.py

@@ -26,9 +26,16 @@ import cx_Freeze
 from cx_Freeze import setup, Executable
 from setuptools import find_packages
 
+if sys.version_info >= (3, 11):
+    import tomllib
+else:
+    import tomli as tomllib
+
 # Discover the version
-with open(os.path.join("..", "cli", "onionshare_cli", "resources", "version.txt")) as f:
-    version = f.read().strip()
+pyproject_filename = os.path.join("..", "cli", "pyproject.toml")
+with open(pyproject_filename, "rb") as f:
+    pyproject = tomllib.load(f)
+    version = pyproject['project']['version']
     # change a version like 2.6.dev1 to just 2.6, for cx_Freeze's sake
     last_digit = version[-1]
     if version.endswith(f".dev{last_digit}"):