docs: Update osx brew install instruction

Summary:

Python 3.x is now the default python formula in homebrew.
https://brew.sh/2018/01/19/homebrew-1.5.0/ has some more info.

Backport of core PR12586
https://github.com/bitcoin/bitcoin/pull/12586/files

Depends on D3908.

Test Plan: Read it, test it.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3909