test_system: relax test_disk_partitions asserts on mountpoints

As on at least Solaris and modern Linux systems they can be files too.
On linux bind mounts may commonly be used with containers.

Fix giampaolo#1573

psutil/tests/test_system.py

@@ -501,11 +501,8 @@ def test_disk_partitions(self):
                 # we cannot make any assumption about this, see:
                 # http://goo.gl/p9c43
                 disk.device
-            if SUNOS or TRAVIS:
-                # on solaris apparently mount points can also be files
-                assert os.path.exists(disk.mountpoint), disk
-            else:
-                assert os.path.isdir(disk.mountpoint), disk
+            # on modern systems mount points can also be files
+            assert os.path.exists(disk.mountpoint), disk
             assert disk.fstype, disk
 
         # all = True