tests: force \n newlines when writing to sys.stdout

Without this, Python 3 on Windows inserts some \r that aren't
present in the input, causing test-http-bad-server.t to fail.
After this change, the test passes on Python 3 on Windows!

Differential Revision: https://phab.mercurial-scm.org/D8341

tests/filtertraceback.py

@@ -4,8 +4,19 b''
 
 from __future__ import absolute_import, print_function
 
+import io
 import sys
 
+if sys.version_info[0] >= 3:
+    # Prevent \r from being inserted on Windows.
+    sys.stdout = io.TextIOWrapper(
+        sys.stdout.buffer,
+        sys.stdout.encoding,
+        sys.stdout.errors,
+        newline="\n",
+        line_buffering=sys.stdout.line_buffering,
+    )
+
 state = 'none'
 
 for line in sys.stdin: