gh-100562: improve performance of `pathlib.Path.absolute()` (GH-100563)

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.

```shell
# 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
```

Automerge-Triggered-By: GH:AlexWaygood