pdb + expect tests failing in Python 3.7

Link to a failing build:

https://github.com/pytest-dev/pytest/runs/6860982658

FAILED testing/test_debugging.py::TestPDB::test_pdb_interaction_capturing_simple - pexpect.exceptions.TIMEOUT: Timeout exceeded.
FAILED testing/test_debugging.py::TestPDB::test_pdb_interaction_capturing_twice - pexpect.exceptions.TIMEOUT: Timeout exceeded.
FAILED testing/test_debugging.py::TestPDB::test_pdb_with_injected_do_debug - assert False
FAILED testing/test_debugging.py::TestDebuggingBreakpoints::test_pdb_not_altered - pexpect.exceptions.TIMEOUT: Timeout exceeded.
FAILED testing/test_debugging.py::test_pdb_suspends_fixture_capturing[capfd] - assert 'out_inner' not in ') c\r\n\r\n...========\r\n'
FAILED testing/test_debugging.py::test_pdb_suspends_fixture_capturing[capsys] - assert 'out_inner' not in ') c\r\n\r\n...========\r\n'

Marking those tests as xfail so they get out of the way for now, but the cause of the errors should be investigated eventually.

Tests marked as xfail in #10043.

Start with the failing build and the listed tests in testing/test_debugging.py, running them under Python 3.7. Review the xfail changes referenced in #10043 and investigate the pdb/expect interaction behind the timeouts and assertions. Done means the affected tests pass without the temporary xfails.