Use standout/reverse instead of yellow for jediUsages

It uses cterm=reverse, which is supported by current Neovim, and at
least for xterm and rxvt-unicode reverse and standout are the same.

See `infocmp`:

> smso=\E[7m / rev=\E[7m

after/syntax/python.vim

@@ -31,4 +31,4 @@ if g:jedi#show_call_signatures > 0 && has('conceal')
     endif
 endif
 
-hi jediUsage term=NONE cterm=NONE ctermfg=None guifg=None gui=NONE ctermbg=3 guibg=Yellow
+hi def jediUsage cterm=reverse gui=standout