_base.py: fix double acquire by TrioExecuto, issue #143

[axiomiety]

Fixes #143

Ran pytest on both master and the branch with the fix - output is the same (there are existing failures):

• on executor-double-acquire-fix

  =============================================== short test summary info ================================================
  FAILED tests/interop/test_calls.py::TestCalls::test_trio_asyncio_cancel_direct - BaseExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
  FAILED tests/test_misc.py::test_keyboard_interrupt_teardown - BaseExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
  FAILED tests/test_misc.py::test_cancel_loop[True] - ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
  FAILED tests/test_misc.py::test_run_trio_task_errors - BaseExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
  FAILED tests/test_trio_asyncio.py::test_cancel_loop_with_tasks[True-False] - ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
  FAILED tests/test_trio_asyncio.py::test_cancel_loop_with_tasks[True-True] - ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
  =========================== 6 failed, 2504 passed, 62 skipped, 4 xfailed in 71.12s (0:01:11) ===========================

• on master

  ================================================ short test summary info ================================================
  FAILED tests/interop/test_calls.py::TestCalls::test_trio_asyncio_cancel_direct - BaseExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
  FAILED tests/test_misc.py::test_keyboard_interrupt_teardown - BaseExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
  FAILED tests/test_misc.py::test_cancel_loop[True] - ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
  FAILED tests/test_misc.py::test_run_trio_task_errors - BaseExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
  FAILED tests/test_trio_asyncio.py::test_cancel_loop_with_tasks[True-False] - ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
  FAILED tests/test_trio_asyncio.py::test_cancel_loop_with_tasks[True-True] - ExceptionGroup: Exceptions from Trio nursery (1 sub-exception)
  =========================== 6 failed, 2504 passed, 62 skipped, 4 xfailed in 71.33s (0:01:11) ============================

[oremanj]

Please add a newsfragment for this change, as well as a test that would have failed before the fix and now doesn't. Once those are set, I should be able to merge this.