Revert "Improves a bit portability"

This reverts commit d054e313.

os.path.join is *not* suitable for concatenating parts of the basename of
a file.

Closes: #11526

libmat/parser.py

@@ -51,7 +51,7 @@ class GenericParser(object):
     def create_backup_copy(self):
         """ Create a backup copy
         """
-        shutil.copy2(self.filename, os.path.join(self.filename, '.bak'))
+        shutil.copy2(self.filename, self.filename + '.bak')
 
     def do_backup(self):
         """ Keep a backup of the file if asked.
@@ -60,7 +60,7 @@ class GenericParser(object):
             but it greatly simplify new strippers implementation.
         """
         if self.backup:
-            shutil.move(self.filename, os.path.join(self.filename, '.bak'))
+            shutil.move(self.filename, self.filename + '.bak')
         else:
             mat.secure_remove(self.filename)
         shutil.move(self.output, self.filename)