Change dict.pop such that KeyError uses toString()

Minor fix from https://hg.python.org/jython/rev/f68d65a141f7, which
was using key.asString() - this was throwing a TypeError for keys that
were not str/unicode
No longer causes a stack overflow on "infinite reloads"

Previously infinite_reload.py in test_import would cause a
RuntimeError due to a stack overflow. Now handles the same as CPython
by tracking which module is currently being reloaded, and returns
immediately with that module.