Fix shebang workaround on Python 3

This fixes up commit d5199b9d
("Add support for Python 3").

slapos/recipe/cmmi/__init__.py

@@ -333,8 +333,8 @@ echo %s
                 if not stat.S_ISREG(st_mode):
                     continue
                 with open(f_abspath, 'rb') as f:
-                    header = f.readline(128)
-                if header.startswith(b'#!') and header[-1] != b'\n':
+                    header = f.readline(128) # 256 starting from Linux 5.1
+                if header.startswith(b'#!') and header[-1:] != b'\n':
                     os.rename(f_abspath, f_abspath + '.shebang')
                     with open(f_abspath, 'w') as f:
                         os.fchmod(f.fileno(), stat.S_IMODE(st_mode))