gh-122989: Replace duplicate “self.policy.linesep” with “linesep” (#123002)

`linesep` is already defined as `self.policy.linesep`.  It appears that previous refactor was not completed.

Author: Damien-Chen

Lib/email/generator.py

@@ -227,7 +227,7 @@ def _write_headers(self, msg):
             folded = self.policy.fold(h, v)
             if self.policy.verify_generated_headers:
                 linesep = self.policy.linesep
-                if not folded.endswith(self.policy.linesep):
+                if not folded.endswith(linesep):
                     raise HeaderWriteError(
                         f'folded header does not end with {linesep!r}: {folded!r}')
                 if NEWLINE_WITHOUT_FWSP.search(folded.removesuffix(linesep)):