bpo-40597: email: Use CTE if lines are longer than max_line_length consistently (gh-20038)

raw_data_manager (default for EmailPolicy, EmailMessage)
does correct wrapping of 'text' parts as long as the message contains
characters outside of 7bit US-ASCII set: base64 or qp
Content-Transfer-Encoding is applied if the lines would be too long
without it.  It did not, however, do this for ascii-only text,
which could result in lines that were longer than
policy.max_line_length or even the rfc 998  maximum.

This changeset fixes the heuristic so that if lines are longer than
policy.max_line_length, it will always apply a
content-transfer-encoding so that the lines are wrapped correctly.

Lib/email/contentmanager.py

@@ -146,13 +146,13 @@ def _encode_text(string, charset, cte, policy):
     def normal_body(lines): return b'\n'.join(lines) + b'\n'
     if cte==None:
         # Use heuristics to decide on the "best" encoding.
-        try:
-            return '7bit', normal_body(lines).decode('ascii')
-        except UnicodeDecodeError:
-            pass
-        if (policy.cte_type == '8bit' and
-                max(len(x) for x in lines) <= policy.max_line_length):
-            return '8bit', normal_body(lines).decode('ascii', 'surrogateescape')
+        if max(len(x) for x in lines) <= policy.max_line_length:
+            try:
+                return '7bit', normal_body(lines).decode('ascii')
+            except UnicodeDecodeError:
+                pass
+            if policy.cte_type == '8bit':
+                return '8bit', normal_body(lines).decode('ascii', 'surrogateescape')
         sniff = embedded_body(lines[:10])
         sniff_qp = quoprimime.body_encode(sniff.decode('latin-1'),
                                           policy.max_line_length)