* On Windows, use subprocess.run() instead of os.execv().
* Only add needed options
* Rename reexec parameter to _add_python_opts.
* Rename --no-reexec option to --dont-add-python-opts.
When --fast-ci or --slow-ci option is used, regrtest now replaces the
current process with a new process to add "-u -W default -bb -E"
options to Python.
Changes:
* PCbuild/rt.bat and Tools/scripts/run_tests.py no longer need to add
"-u -W default -bb -E" options to Python: it's now done by
regrtest.
* Fix Tools/scripts/run_tests.py: flush stdout before replacing the
process. Previously, buffered messages were lost.
* Rename libregrtest.main_in_temp_cwd() to libregrtest.main()
* Add regrtest.main_in_temp_cwd() alias to libregrtest.main()
* Move old main_in_temp_cwd() code into libregrtest.Regrtest.main()
* Update multiple scripts to call libregrtest.main()