tests: force `dummysmtpd.py` to write its log file with '\n' on Windows

The log files were being `cat'd` in `test-patchbomb-tls.t`, and causing
gratuitous failures.  Since `sys.stdout` is being written to with `str` instead
of `bytes`, use a `io.TextIOWrapper` to change the EOL, like 2924676d4728.

tests/dummysmtpd.py

@@ -3,6 +3,7 b''
 """dummy SMTP server for use in tests"""
 
 
+import io
 import optparse
 import os
 import socket
@@ -16,6 +17,13 b' from mercurial import ('
     ui as uimod,
 )
 
+if pycompat.iswindows:
+    sys.stdout = io.TextIOWrapper(
+        sys.stdout.buffer,
+        sys.stdout.encoding,
+        sys.stdout.errors,
+        newline="\n",
+    )
 
 if os.environ.get('HGIPV6', '0') == '1':
     family = socket.AF_INET6