Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
EDIT : This build can be seen as more of a farming build. The splash damage from the Tiamat combined with Teemo's attack speed instantly helps destroying creeps and farming plenty of gold. You can use the mushrooms that you would usually use for farming to make even more traps for more kills. Also, the splash damage from the Tiamat would hit most of the enemies in team fights. Even though the damage is not that high, you will be able to rack up assists when the enemies are killed, giving you even more gold. This will help you to go ahead and buy Infinity edges as fast as you can.
This build was made to mainly show what items to buy for teemo. This item build has the best combination of attack damage and speed to get the most kills from teemo's autoattack. Skill points should be trying to max out toxic shot and blinding dart while having one point in move quick. Of course the ulti should be leveled whenever possible.