Finish the method definition given below.
Given a string str representing a ribonucleic acid (RNA) strand, the method rna_complement should return the reverse complement of the string.
The str RNA will be represented as a string containing only the characters "A", "C", "G" and "U". Since RNA can only (canonically) allow pairings of A/U and G/C, the complement of an RNA would be as follows:
- 'AAA' would be 'UUU' and 'UUU' would be 'AAA'
- 'GGG' would be 'CCC' and 'CCC' would be 'GGG'
Finally the function should reverse the string obtained till now by complementing RNA and return it.