2to3: Remove xreadlines and replace f.readlines() by f where valid.

An open file `f` has been an iterator since python2.3 and
`f.xreadlines()` is no longer needed, so replace it with `f`. Also
replace `f.readlines()` with `f` where an iterator will do. The
replacement of `f.readlines()` is not critical because it is a list in
both python2 and python3, but the code is a bit cleaner.

Closes #3093
Merge pull request #3121 from charris/2to3-fix-for-in-f_dot_readlines

2to3: Remove xreadlines and replace f.readlines() by f where valid.