Вариант 2

F = open('file.txt', 'r')
f = F.read()
circ = ''

def check(L):
  k = 1
  mx = 0
  a = len(L) - 1
  for i in range(a-1):
    if ord(L[i]) == ord(L[i+1]) or ord(L[i]) == ord(L[i+1])-1:
      k += 1
    else:
      k = 1
    if k > mx:
      mx = k
  return mx

mx = 0
a = len(f)
for i in range(a):
    for c in range(a):
        if f[i] == f[c]:
            for n in range(i, c+1):
                lettter = f[n]
                circ += f[n]
            if len(circ) == 1:
                circ = ''
                continue
            circ = circ*2
            print(circ)
            print(check(circ))
            if check(circ) > mx:
              mx = check(circ)
            circ = ''

print('mx:', mx)
F.close()