macOS: use ~/.config/pip/pip.conf as default as well

[mhinz]

• Pip version: 9.0.1
• Python version: 3.5.2
• Operating System: macOS Sierra (10.12.1)

Description:

According to the docs the new per-user default configuration location is at ~/Library/Application Support/pip/pip.conf.

But I think even on macOS the default location should be ~/.config/pip/pip.conf, even when no $XDG_CONFIG_HOME is set. Or at least it should be read if ~/Library/Application Support/pip/pip.conf doesn't exist. Currently this doesn't happen.

Actually pip would be the first program forcing me to put a config file under ~/Library. So, please handle Unix and macOS the same in this regard.

FWIW, other software (git, fish, neovim) handle it that way as well.