Fix Py3 Incompatable str + bytes issue. (#456)

Fix Py3 Incompatable str + bytes issue.

buildozer/__init__.py

@@ -825,10 +825,10 @@ class Buildozer(object):
         if not self.file_exists(main_py):
             #self.error('Unable to patch main_py to add applibs directory.')
             return
-
-        header = ('import sys, os; '
-                  'sys.path = [os.path.join(os.getcwd(),'
-                  '"..", "_applibs")] + sys.path\n')
+        
+        header = (b'import sys, os; '
+                   b'sys.path = [os.path.join(os.getcwd(),'
+                   b'"..", "_applibs")] + sys.path\n')
         with open(main_py, 'rb') as fd:
             data = fd.read()
         data = header + data