gh-100562: improve performance of pathlib.Path.absolute()

[barneygale]

Increase performance of the absolute() method by calling os.getcwd() directly, rather than using the Path.cwd() class method. This avoids constructing an extra Path object (and the parsing/normalization that comes with it).

Decrease performance of the cwd() class method by calling the Path.absolute() method, rather than using os.getcwd() directly. This involves constructing an extra Path object. We do this to maintain a longstanding pattern where os functions are called from only one place, which allows them to be more readily replaced by users. As cwd() is generally called at most once within user programs, it's a good bargain.

# before
$ ./python -m timeit -s 'from pathlib import Path; p = Path("foo", "bar")' 'p.absolute()'
50000 loops, best of 5: 9.04 usec per loop
# after
$ ./python -m timeit -s 'from pathlib import Path; p = Path("foo", "bar")' 'p.absolute()'
50000 loops, best of 5: 5.02 usec per loop

• Issue: Improve performance of pathlib.Path.absolute() #100562