remove os.sep as it behaves differently linux and windows.

* on linux it is '/'

* on windows it is '\'

Author: srinivasreddy

_pytest/config.py

@@ -1042,8 +1042,8 @@ def _mark_plugins_for_rewrite(self, hook):
         )
 
         for fn in package_files:
-            is_simple_module = os.sep not in fn and fn.endswith('.py')
-            is_package = fn.count(os.sep) == 1 and fn.endswith('__init__.py')
+            is_simple_module = '/' not in fn and fn.endswith('.py')
+            is_package = fn.count('/') == 1 and fn.endswith('__init__.py')
             if is_simple_module:
                 module_name, ext = os.path.splitext(fn)
                 hook.mark_rewrite(module_name)
@@ -1351,7 +1351,7 @@ def determine_setup(inifile, args, warnfunc=None):
                 rootdir, inifile, inicfg = getcfg(dirs, warnfunc=warnfunc)
                 if rootdir is None:
                     rootdir = get_common_ancestor([py.path.local(), ancestor])
-                    is_fs_root = os.path.splitdrive(str(rootdir))[1] == os.sep
+                    is_fs_root = os.path.splitdrive(str(rootdir))[1] == '/'
                     if is_fs_root:
                         rootdir = ancestor
     return rootdir, inifile, inicfg or {}