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

file() is not present in Python 3.
This patch also adds a b'' prefix to make sure we write bytes in Python 3.

Differential Revision: https://phab.mercurial-scm.org/D2126

tests/test-import-bypass.t

@@ -227,7 +227,7 b' Test patch.eol is handled'
 (this also tests that editor is not invoked for '--bypass', if the
 commit message is explicitly specified, regardless of '--edit')
 
-  $ $PYTHON -c 'file("a", "wb").write("a\r\n")'
+  $ $PYTHON -c 'open("a", "wb").write(b"a\r\n")'
   $ hg ci -m makeacrlf
   $ HGEDITOR=cat hg import -m 'should fail because of eol' --edit --bypass ../test.diff
   applying ../test.diff