Modify the way to install python3 and boost-python

[hsiaoyi0504]

Few updates:

• use boost-python3 in python3-based installation because they are different formulas now.
  • https://github.com/Homebrew/homebrew-core/blob/master/Formula/boost-python.rb
  • https://github.com/Homebrew/homebrew-core/blob/master/Formula/boost-python3.rb
• numpy doesn't have with-python3 option anymore.
  • https://github.com/Homebrew/homebrew-core/blob/master/Formula/numpy.rb
• python formula is for python3 and python@2 is for python2. See #55.

[hsiaoyi0504]

Let the CI finishes first.