Prevent use of Database and Collection in boolean expressions

Any undefined attribute access on MongoClient gives you a Database, and Python always considers a Database "true":

if client.pirmary:  # Oops.

    print("I'm connected!")

Database and Collection act this way too, leading to mistakes like the one we corrected here:

https://github.com/mongodb/mongo-python-driver/commit/73edd2c134f43d58531e3dc26499b777354fb7ea

Override __nonzero__ and __bool__ for Database and Collection to raise NotImplementedError, preventing accidental use of these objects as bools in Python 2 and 3, respectively.

This is a breaking change for code like this:

db = None

if some_test():

    db = get_a_database()

if db:

    print("We got a Database")

It would need to be rewritten:

if db is not None: