fix(python): leftover jsii-kernel-* directories in TMPDIR (#2100)

The Python runtime did not properly signal shutdown to the `node`
process hosting the *jsii kernel*, causing it to be abruptly killed on
Python interpreter exit. This means the *jsii kernel*'s own clean-up
code was not allowed to run.

This adds an `atexit` hook which will properly close the `node` process'
`STDIN` file descriptor, effectively signaling it should shut down. This
then gives the process 5 seconds to clean up after itself and finish
before sending it `SIGTERM` (at which point it is given 5 seconds to
terminate after which it will be terminated).

packages/@jsii/python-runtime/src/jsii/_kernel/providers/process.py

@@ -1,3 +1,4 @@
+import atexit
 import datetime
 import contextlib
 import enum
@@ -275,16 +276,26 @@ def start(self):
             stdout=subprocess.PIPE,
             env=environ,
         )
+
+        # Clean this process up at exit, so it terminates "gracefully"
+        atexit.register(self.stop)
+
         self.handshake()
 
     def stop(self):
-        # TODO: We can write an empty string here instead?
-        self._process.terminate()
+        # This process is closing already, un-registering the hook to not fire twice
+        atexit.unregister(self.stop)
+
+        # Close the process' STDIN, singalling we are done with it
+        self._process.stdin.close()
 
         try:
             self._process.wait(timeout=5)
         except subprocess.TimeoutExpired:
-            self._process.kill()
+            try:
+                self._process.terminate(timeout=5)
+            except subprocess.TimeoutExpired:
+                self._process.kill()
 
         self._ctx_stack.close()