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

cli/onionshare_cli/common.py

@@ -19,6 +19,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 """
 import base64
 import hashlib
+import importlib.metadata as importlib_metadata
 import importlib.resources as importlib_resources
 import os
 import platform
@@ -59,8 +60,7 @@ class Common:
             self.platform = "BSD"
 
         # The current version of OnionShare
-        with open(self.get_resource_path("version.txt")) as f:
-            self.version = f.read().strip()
+        self.version = importlib_metadata.version(__package__)
 
     def display_banner(self):
         """