It is a clear question but the problem here seems to be that you are reading from the “beta” release documentation but in all likelihood you actually at most have “pymongo” 2.8 installed rather than the “3.0b” referred to in the link you quote.
The 2.8 release tutorial points to the .insert() method instead:
posts.insert({'a':1})
Since .insert_one() is only available in the 3.0b driver.
Either force the installation of the “beta” driver or live with a stable driver and the available methods.
This seems to be the fault of the current “search engine response” matching the “beta release” as “current”.