py3: make sure we open file in bytes mode

Differential Revision: https://phab.mercurial-scm.org/D3274

mercurial/sparse.py

@@ -579,7 +579,7 b' def importfromfiles(repo, opts, paths, f'
         # part of the active rules.
         changed = False
         for p in paths:
-            with util.posixfile(util.expandpath(p)) as fh:
+            with util.posixfile(util.expandpath(p), mode='rb') as fh:
                 raw = fh.read()
 
             iincludes, iexcludes, iprofiles = parseconfig(repo.ui, raw)