bpo-44395: Fix MIMEPart.as_string to pass unixfrom properly (GH-26685)

(cherry picked from commit 30f7a77f359a0fc6e37988b0f317a77a15d66b7b)

Co-authored-by: Dong-hee Na <donghee.na@python.org>
diff --git a/Lib/email/message.py b/Lib/email/message.py
index 3701b30..db30d9a 100644
--- a/Lib/email/message.py
+++ b/Lib/email/message.py
@@ -948,7 +948,7 @@ def __init__(self, policy=None):
         if policy is None:
             from email.policy import default
             policy = default
-        Message.__init__(self, policy)
+        super().__init__(policy)
 
 
     def as_string(self, unixfrom=False, maxheaderlen=None, policy=None):
@@ -965,7 +965,7 @@ def as_string(self, unixfrom=False, maxheaderlen=None, policy=None):
         policy = self.policy if policy is None else policy
         if maxheaderlen is None:
             maxheaderlen = policy.max_line_length
-        return super().as_string(maxheaderlen=maxheaderlen, policy=policy)
+        return super().as_string(unixfrom, maxheaderlen, policy)
 
     def __str__(self):
         return self.as_string(policy=self.policy.clone(utf8=True))