bpo-34384: Fix os.readlink() on Windows (GH-8740)

bpo-34384: Fix os.readlink() on Windows (GH-8740)

os.readlink() now accepts path-like and bytes objects on Windows.
Previously, support for path-like and bytes objects was only
implemented on Unix.

This commit also merges Unix and Windows implementations of
os.readlink() in one function and adds basic unit tests to increase
test coverage of the function.