Currently, the source transformer inserts
from mercurial.pycompat import delattr, getattr, hasattr, setattr, open, unicode
to the top of every file. As part of getting rid of the source transformer,
we'll need to have source code call these wrappers directly. Rather than
rewrite all call sites to call pycompat.*, I think it makes sense to import
needed symbols via explicit imports. That requires loosening the import checker
to allow this.