It looks like logging a warning at Python exit always works on Python 2.7:
---
import _warnings

class Bla:
    def __del__(self, w=_warnings):
        w.warn_explicit('message', DeprecationWarning, 'x.py', 5)

bla = Bla()
---

It looks like it uses the Python implementation and linecache.getline() always works. Maybe the warning is emitted earlier that the Python 3 code emitting ResourceWarning. I see that warnings.py of Python 2 imports linecache at top level, whereas the Python 3 code imports the module at the first use in showwarning(). I recall that imports at top level are avoided to get faster startup time.