bpo-41373: IDLE: Fix saving files loaded with no newlines or mixed newlines (GH-21597)

Fixes regression in 3.8.4 and 3.9.0b4.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
(cherry picked from commit 0dd463c8a4)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Lib/idlelib/iomenu.py

@@ -155,6 +155,17 @@ class IOBinding:
                                    parent=self.text)
             return False
 
+        if not isinstance(eol_convention, str):
+            # If the file does not contain line separators, it is None.
+            # If the file contains mixed line separators, it is a tuple.
+            if eol_convention is not None:
+                tkMessageBox.showwarning("Mixed Newlines",
+                                         "Mixed newlines detected.\n"
+                                         "The file will be changed on save.",
+                                         parent=self.text)
+                converted = True
+            eol_convention = os.linesep  # default
+
         self.text.delete("1.0", "end")
         self.set_filename(None)
         self.fileencoding = fileencoding

Misc/NEWS.d/next/IDLE/2020-07-24-17-49-58.bpo-41373.YQIPu_.rst

@@ -0,0 +1,3 @@
+Save files loaded with no line ending, as when blank, or different line
+endings, by setting its line ending to the system default. Fix regression in
+3.8.4 and 3.9.0b4.