GH-103548: Improve performance of pathlib.Path.[is_]absolute()

[barneygale]

Pass the unnormalized path to os.path.isabs():

$ ./python -m timeit -s 'from pathlib import Path; p = Path("/etc")' 'p.is_absolute()'
1000000 loops, best of 5: 375 nsec per loop  # before
1000000 loops, best of 5: 240 nsec per loop  # after

When making an empty path absolute, avoid calling os.path.join(), and prevent pathlib from later re-normalizing the result of os.getcwd() by storing it as path._str:

$ ./python -m timeit -s 'from pathlib import Path' 'str(Path.cwd())'
50000 loops, best of 5: 9.1 usec per loop    # before
100000 loops, best of 5: 3.73 usec per loop  # after

• Issue: Improve performance of pathlib.Path.[is_]absolute() #103548

[AlexWaygood]

LGTM!