The previous `_parse_args()` method pulled the `_parts` out of any
supplied `PurePath` objects; these were subsequently joined in
`_from_parts()` using `os.path.join()`. This is actually a slower form of
joining than calling `fspath()` on the path object, because it doesn't take
advantage of the fact that the contents of `_parts` is normalized!

This reduces the time taken to run `PurePath("foo", "bar") by ~20%, and
the time taken to run `PurePath(p, "cheese")`, where
`p = PurePath("/foo", "bar", "baz")`, by ~40%.

Co-authored-by: Alex Waygood <[email protected]>

…lk6mt.rst

Co-authored-by: Alex Waygood <[email protected]>