py3: replace file() with open() in test-import-git.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/D2124

tests/test-import-git.t

@@ -563,10 +563,10 b' Simulate a copy/paste turning LF into CR'
   > Mc$`b*O5$Pw00T?_*Z=?k
   > 
   > EOF
-  >>> fp = file('binary.diff', 'rb')
+  >>> fp = open('binary.diff', 'rb')
   >>> data = fp.read()
   >>> fp.close()
-  >>> file('binary.diff', 'wb').write(data.replace('\n', '\r\n'))
+  >>> open('binary.diff', 'wb').write(data.replace(b'\n', b'\r\n'))
   $ rm binary2
   $ hg import --no-commit binary.diff
   applying binary.diff