count only letter abcdefghijklmnopqrstuvwxyz in a list : How to count only letter abcdefghijklmnopqrstuvwxyz in a list in alphabetical order without "Counter"??

    For example
    input 
   

    ['Bro', 'lo', '27', 'b']

   output
   

    [['b', 1], ['l', 1], ['o', 2], ['r', 1]]

