Tests fail with pypy3

Hi, I want to to use ptpython with pypy3. I also can install and use it, but the tests of python-prompt-toolkit do not run:

 * pypy3: running distutils-r1_run_phase python_test
===================================================================== test session starts =====================================================================
platform linux -- Python 3.6.9[pypy-7.3.1-final], pytest-6.1.2, py-1.9.0, pluggy-0.13.1 -- /usr/bin/pypy3
cachedir: .pytest_cache
rootdir: /var/tmp/portage/dev-python/prompt_toolkit-3.0.7/work/prompt_toolkit-3.0.7
collecting ... collected 136 items

tests/test_async_generator.py::test_generator_to_async_generator PASSED                                                                                 [  0%]
tests/test_buffer.py::test_initial PASSED                                                                                                               [  1%]
tests/test_buffer.py::test_insert_text PASSED                                                                                                           [  2%]
tests/test_buffer.py::test_cursor_movement PASSED                                                                                                       [  2%]
tests/test_buffer.py::test_backspace PASSED                                                                                                             [  3%]
tests/test_buffer.py::test_cursor_up PASSED                                                                                                             [  4%]
tests/test_buffer.py::test_cursor_down PASSED                                                                                                           [  5%]
tests/test_buffer.py::test_join_next_line PASSED                                                                                                        [  5%]
tests/test_buffer.py::test_newline PASSED                                                                                                               [  6%]
tests/test_buffer.py::test_swap_characters_before_cursor PASSED                                                                                         [  7%]
tests/test_cli.py::test_simple_text_input PASSED                                                                                                        [  8%]
tests/test_cli.py::test_emacs_cursor_movements PASSED                                                                                                   [  8%]
tests/test_cli.py::test_emacs_kill_multiple_words_and_paste PASSED                                                                                      [  9%]
tests/test_cli.py::test_interrupts ^Csandbox:stop  caught signal 2 in pid 2054

The test_interrupts test does not end (I have waited several minutes before aborting).

Other CPython implementations work fine here.

Start with tests/test_cli.py, especially test_interrupts, and run the test suite under pypy3 to reproduce the hang. Compare its behavior with the other Python implementations mentioned in the report. Done means test_interrupts completes without requiring an abort and the full test suite passes under pypy3.