Use byte literals instead of .encode()

This is a suggestion from pyupgrade:
https://github.com/asottile/pyupgrade#encode-to-bytes-literals

Fixed by running `ruff --select UP012 --fix .`:
UP012 [*] Unnecessary call to `encode` as UTF-8

setuptools/command/sdist.py

@@ -189,7 +189,7 @@ def _manifest_is_not_generated(self):
 
         with open(self.manifest, 'rb') as fp:
             first_line = fp.readline()
-        return first_line != '# file GENERATED by distutils, do NOT edit\n'.encode()
+        return first_line != b'# file GENERATED by distutils, do NOT edit\n'
 
     def read_manifest(self):
         """Read the manifest file (named by 'self.manifest') and use it to