After the research of tools, I have found this tool to fit my usecase: pysamstats.

It gives me what I need: for every pileup position, I can get the number of A, C, G, T, insertions and deletions.

import pysam
import pysamstats

bamfile = pysam.AlignmentFile(bam_file_path)  
for record in pysamstats.stat_variation(bamfile, chrom=chrom, fafile=ref_path):
    print(record['pos'])
    print(record['A'])
    print(record['C'])
    print(record['G'])
    print(record['T'])
    print(record['insertions'])
    print(record['deletions'])

Thanks for your answers.