avoiding python@2 redundant dependency
There is no getting around the pain of managing dependencies. What you are seeing is that for many, many years python 2 was the standard and as we finally are reaching the end of support, some major frameworks are making the move after preparing for years to do so.  For the next 6 months, I would expect most active or easy to port projects to move to python 3. Time will tell of course. 

If you feel you want or need to force your transition to python 3 in homebrew, I would try first removing it so you can validate you actually need it.

brew rm python@2


If you do have it as a dependency, you probably can do nothing and just give up a little space and a little time as every package you care about gets updated to use python 3. Set your path to use the version you want and just let it fade into history.