[3.9] bpo-43757: Make pathlib use os.path.realpath() to resolve symlinks in a path...

[3.9] bpo-43757: Make pathlib use os.path.realpath() to resolve symlinks in a path (GH-25264) (GH-135035)

Also adds a new "strict" argument to realpath() to avoid changing the default behaviour of pathlib while sharing the implementation.

(cherry-picked from commit baecfbd849dbf42360d3a84af6cc13160838f24d)

Co-authored-by: Barney Gale <barney.gale@gmail.com>