If a vim/neovim user wishes to suppress loading the vim plugin by
setting g:load_black in their VIMRC (for me, Arch linux automatically
adds the plugin to Neovim's RTP, even though I'm not using it), the
current location of the test comes after a call to has('python3'). This
adds, in my tests, between 35 and 45 ms to Vim load time (which I know
isn't a lot but it's also unnecessary). Moving the call to
`exists('g:load_black')` to before the call to `has('python3')` removes
this unnecessary test and speeds up loading.
Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>