you should have your ffmpeg path into the environment variables first.

import os
from re import match
path='E:\\blender tutorials\\' #you should change the directory before you run this code.
files=os.listdir(path)
for f  in files:
    if match('.+\.flv$',f):
        outf=f.replace('.flv','.mp4') #change mp4 to avi if you want avi files.
        os.system(r'ffmpeg.exe -i "%s" "%s"'%(path+f,path+outf))