packaging: leverage os.path.relpath() in setup.py

`os.path.relpath()` has existed since Python 2.6, so we can safely use
it. This fixes a bug in the current code when the common prefix is "/"
(in which case `uplevel` would be one less than it should).

Differential Revision: https://phab.mercurial-scm.org/D7875

setup.py

@@ -1064,11 +1064,7 b' class hginstallscripts(install_scripts):'
             # absolute path instead
             libdir = self.install_lib
         else:
-            common = os.path.commonprefix((self.install_dir, self.install_lib))
-            rest = self.install_dir[len(common) :]
-            uplevel = len([n for n in os.path.split(rest) if n])
-
-            libdir = uplevel * ('..' + os.sep) + self.install_lib[len(common) :]
+            libdir = os.path.relpath(self.install_lib, self.install_dir)
 
         for outfile in self.outfiles:
             with open(outfile, 'rb') as fp: