py3: replace file() with open() in test-import.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/D2123
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])
http://example.com/, branches: (None, [])
http://example.com/, branches: (None, [])
http://example.com/, branches: ('foo', [])