Move signal handler attachment back to main body. Signals cannot be

attached from threads in Python. While it is impossible to guarantee
that any piece of Python code is not called from a thread (one can always
import this class from the run method of a thread), this reduces the
likelihood of that happening. This commit also adds a clear error message
stating that psij has to be imported from the main thread.

Author: hategan

src/psij/executors/local.py

@@ -18,10 +18,17 @@
 logger = logging.getLogger(__name__)
 
 
+if threading.current_thread() != threading.main_thread():
+    raise ImportError('The psij module must be imported from the main thread.')
+
+
 def _handle_sigchld(signum: int, frame: Optional[FrameType]) -> None:
     _ProcessReaper.get_instance()._handle_sigchld()
 
 
+signal.signal(signal.SIGCHLD, _handle_sigchld)
+
+
 _REAPER_SLEEP_TIME = 0.2