handle SystemError exception

[rathann]

Fixes the following test failures when building with Python3.14:

=========================== short test summary info ============================
FAILED tests/runners.py::Local_::pty::when_pty_True_we_use_pty_fork_and_os_exec - SystemError: buffer overflow
FAILED tests/runners.py::Local_::pty::pty_uses_WEXITSTATUS_if_WIFEXITED - SystemError: buffer overflow
FAILED tests/runners.py::Local_::pty::pty_uses_WTERMSIG_if_WIFSIGNALED - SystemError: buffer overflow
FAILED tests/runners.py::Local_::pty::WTERMSIG_result_turned_negative_to_match_subprocess - SystemError: buffer overflow
FAILED tests/runners.py::Local_::pty::pty_is_set_to_controlling_terminal_size - SystemError: buffer overflow
FAILED tests/runners.py::Local_::pty::spurious_OSErrors_handled_gracefully - SystemError: buffer overflow
FAILED tests/runners.py::Local_::pty::other_spurious_OSErrors_handled_gracefully - SystemError: buffer overflow
FAILED tests/runners.py::Local_::pty::non_spurious_OSErrors_bubble_up - SystemError: buffer overflow
FAILED tests/runners.py::Local_::pty::stop_mutes_errors_on_pty_close - SystemError: buffer overflow
FAILED tests/runners.py::Local_::pty::fallback::can_be_overridden_by_kwarg - SystemError: buffer overflow
FAILED tests/runners.py::Local_::pty::fallback::can_be_overridden_by_config - SystemError: buffer overflow
FAILED tests/runners.py::Local_::pty::fallback::overridden_fallback_affects_result_pty_value - SystemError: buffer overflow
FAILED tests/runners.py::Local_::shell::defaults_to_bash_or_cmdexe_when_pty_True - SystemError: buffer overflow
FAILED tests/runners.py::Local_::shell::may_be_overridden_when_pty_True - SystemError: buffer overflow
FAILED tests/runners.py::Local_::env::uses_execve_for_pty_True - SystemError: buffer overflow
FAILED tests/terminals.py::terminals::pty_size::calls_fcntl_with_TIOCGWINSZ - SystemError: buffer overflow
================== 16 failed, 952 passed, 11 skipped in 6.67s ==================

Resolves #1038 .