Homebrew changed default Python to python 3!

[INADA Naoki]

FYI, they reverted python->python3 symlink. python command is now
Python 2 again.
https://discourse.brew.sh/t/python-and-pep-394/1813

Even though this revert, it is significant step:

* many formulas dropped `depends_on "python"`.  Python 2 was installed
often by dependency before
  but it's rare for now.

* Some formulas (including vim) keep `depends_on "python".
  It means they are depending on Python 3 instead of 2.
  It makes vim plugin authors can consider about dropping Python 2 support.
   (vim is linked with Python 3 on recent Ubuntu already)

I hope Apple will provide "/usr/bin/python2" at least in next macOS,
to allow people using "python2" shebang
for scripts support only Python 2.
(And I hope Python 3.6 (or 3.7) is pre-installed in next macOS)...