Kill in-flight tools instantly on quit via killpg(self)

When eris quits, _python_exit joins executor threads, which block on
in-flight subprocess.run calls for up to TIMEOUT seconds. After saving
state, kill the process group to reap all tool subprocesses instantly.
Isolate editor/xdg-open in their own sessions so they survive.

eris/__main__.py

@@ -791,7 +791,8 @@ class Screen:
             self._log.log_message([in_green("Editing "), path_colored,
                                    in_green(f" at line {line_num}…")])
             subprocess.Popen(f"{self.editor_command} +{line_num} {path}", shell=True,
-                             stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+                             stdout=subprocess.PIPE, stderr=subprocess.PIPE,
+                             start_new_session=True)
     def patch_file(self):
         result = self._summary.get_selection()
         if hasattr(result.tool, "patch_command") and result.tool.patch_command is not None:
@@ -849,7 +850,8 @@ class Screen:
         path = self._summary.get_selection().path
         path_colored = lscolors.path_colored(path)
         self._log.log_message([in_green("Opening "), path_colored, in_green("…")])
-        subprocess.Popen(["xdg-open", path], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+        subprocess.Popen(["xdg-open", path], stdout=subprocess.PIPE, stderr=subprocess.PIPE,
+                         start_new_session=True)
 
     def save(self):
         worker.Worker.unsaved_jobs_total = 0
@@ -1077,6 +1079,8 @@ async def main(title, root_path, worker_count=None, editor_command=None, theme=N
         notifier.stop()
     if summary.is_loaded:
         screen.save()
+    if sys.stdout.isatty() and os.getpgrp() == os.getpid():
+        os.killpg(os.getpgrp(), signal.SIGKILL)
 
 
 @contextlib.contextmanager