bpo-28441: Ensure `.exe` suffix in `sys.executable` on MinGW and Cygwin (GH-4348)

This is needed to even the run the test suite on buildbots for affected platforms; e.g.:

```
./python.exe  ./Tools/scripts/run_tests.py -j 1 -u all -W --slowest --fail-env-changed --timeout=11700 -j2
/home/embray/src/python/test-worker/3.x.test-worker/build/python -u -W default -bb -E -W error::BytesWarning -m test -r -w -j 1 -u all -W --slowest --fail-env-changed --timeout=11700 -j2
Traceback (most recent call last):
  File "./Tools/scripts/run_tests.py", line 56, in <module>
    main(sys.argv[1:])
  File "./Tools/scripts/run_tests.py", line 52, in main
    os.execv(sys.executable, args)
PermissionError: [Errno 13] Permission denied
make: *** [Makefile:1073: buildbottest] Error 1
```