py3: replace file() with open() in test-encoding.t

file() is not present in Python 3.
This also makes sure we write things in bytes mode in Python 3.

Differential Revision: https://phab.mercurial-scm.org/D2130

tests/test-encoding.t

@@ -15,9 +15,9 b' we need a repo with some legacy latin-1 '
   $ hg co
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ $PYTHON << EOF
-  > f = file('latin-1', 'w'); f.write("latin-1 e' encoded: \xe9"); f.close()
-  > f = file('utf-8', 'w'); f.write("utf-8 e' encoded: \xc3\xa9"); f.close()
-  > f = file('latin-1-tag', 'w'); f.write("\xe9"); f.close()
+  > f = open('latin-1', 'wb'); f.write(b"latin-1 e' encoded: \xe9"); f.close()
+  > f = open('utf-8', 'wb'); f.write(b"utf-8 e' encoded: \xc3\xa9"); f.close()
+  > f = open('latin-1-tag', 'wb'); f.write(b"\xe9"); f.close()
   > EOF
 
 should fail with encoding error