Bug #1445901: os.path.realpath() is available on Win/Mac too.

Doc/lib/libposixpath.tex

@@ -175,8 +175,8 @@ change the meaning of the path if it contains symbolic links!
 
 \begin{funcdesc}{realpath}{path}
 Return the canonical path of the specified filename, eliminating any
-symbolic links encountered in the path.
-Availability:  \UNIX.
+symbolic links encountered in the path (if they are supported by the
+operating system).
 \versionadded{2.2}
 \end{funcdesc}