Check for existence of breakpoint in builtins to determine if version of Python implements PEP553 https://www.python.org/dev/peps/pep-0553/#implementation this means that if PyPy/PyPy3 etc. later decide to do so we're not coupling to CPython. Add a "supports_breakpoint_builtin" module-level attribute in pytest.debugging
The custom Pdb module should only be set as the system breakpointhook when the user hasn't set PYTHONBREAKPOINT. Otherwise the user may have wanted to override the debugger using the PYTHONBREAKPOINT environment variable.