input: Write a function to calculate the percentage of transcripts mapping to rRNA for a given HTseq_counts file and a corresponding GTF file. The function should return a tuple containing the total number of rRNA transcripts, the total number of all transcripts, and the percentage of rRNA transcripts.
  
  def calculate_rRNA_percentage(htseq_file, gtf_file):
      # Implementation of the function
  
  assert calculate_rRNA_percentage("C17_3_S20_Aligned.out.srt.bam_htseq_combined_counts.txt", "/Users/malwina/Documents/CTR-Data/genomes/Mus_musculus/mm10/Mus_musculus.GRCm38.84.gtf") == (100, 1000, 10.0)
output: def calculate_rRNA_percentage(htseq_file, gtf_file):
      rRNA_transcripts = 0
      total_transcripts = 0
      
      with open(gtf_file, 'r') as gtf:
          for line in gtf:
              if line.startswith('#'):
                  continue
              fields = line.split('\t')
              if fields[2] == 'gene' and 'rRNA' in fields[8]:
                  rRNA_transcripts += 1
      
      with open(htseq_file, 'r') as htseq:
          for line in htseq:
              if not line.startswith('__'):
                  total_transcripts += 1
      
      percentage = (rRNA_transcripts / total_transcripts) * 100
      return (rRNA_transcripts, total_transcripts, percentage)