Move the weighted gas price strategy under `web3.tools`

related to #1463

### What was wrong?

I believe that the weighted gas price strategy should be deprecated or moved to live under `web3.tools`. This is based on the high subjectivity of gas prices which I believe means that we will be unable to supply gas price strategies that work well across the many disparate use cases that Web3.py is intended for.

### How can it be fixed?

The most extreme approach would be to deprecate and remove it. I'm not convinced this is the right choice.

I believe that the following course of action would be appropriate.

1. Move the weighted gas price strategy to live somewhere under `web3.tools`. This serves to designate this as not being part of the core library API, and thus to not be subject to as stringent of requirements for things like breaking changes.
2. Review our documentation around gas price strategies. Make sure that it is sufficiently informative so that someone could write their own gas pricing strategy that would serve their own needs.