While researching PYTHON-1964 I discovered that json_util imports from pymongo.errors (and has for a few years). It really shouldn't. The pymongo package should depend on bson, not the other way around:

~/work/mongo-python-driver/bson $ git grep pymongo
_cbsonmodule.c:     * pymongo.database.Database returns a new Collection instance for
json_util.py:from pymongo.errors import ConfigurationError
objectid.py:        # pickled with pymongo-1.9 or older.
lines 1-3/3 (END)

The bson package has its own errors module. We can define a ConfigurationError there instead.