Closes #29688

Objects without a parent are not necessarily cleaned up in PyQt when their reference count reaches zero, and must be explicitly cleaned up with deleteLater()

This prevents the notifier firing after the signal handling was supposed to have been cleaned up, which can cause spurious errors to be printed if it runs upon further interrupts or at interpreter shutdown.

I've tested this on Linux with Python 3.13, using both of

matplotlib.use('QtAgg') 
matplotlib.use('Qt5Agg') 

Previously, running the following with python -i script.py:

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], [4, 5, 6])
plt.show()

and then pressing ctrl-C in the terminal whilst the figure was shown, would cause a traceback and error message OSError: [Errno 9] Bad file descriptor to be printed as the interpreter exited (by e.g. subsequently pressing ctrl-D), and at least for me, cause the controlling terminal to exit for some reason.

After this change interpreter shutdown appears clean.

Similarly, as in the original bug report, running ipython and then entering in the REPL:

import matplotlib.pyplot as plt
plt.plot([1, 2, 3],[5, 4, 6])
plt.pause(10)

and pressing ctrl-C would result in an infinite loop of OSError: [Errno 9] Bad file descriptor error messages, and with this change results in a single KeyBoardInterrupt traceback as expected.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
  - [] Plotting related features are demonstrated in an example N/A
  - [ ] New Features and API Changes are noted with a directive and release note N/A
  - [] Documentation complies with general and docstring guidelines N/A

I've checked the docs build and the tests run - one test fails: test_font_heuristica[pdf] but it fails regardless of this change. I have only tested on Linux with Python 3.13.3.