Buggy python submission (verdict: Time Limit Exceeded):
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
sys.setrecursionlimit(10 ** 7)

from copy import deepcopy

h, w = map(int, readline().split())
s = [readline().rstrip().decode() for _ in range(h)]
move = [(0, 1), (0, -1), (1, 0), (-1, 0)]
sg = []
memo = [[False] * w for _ in range(h)]
for i in range(h):
    for j in range(w):
        if s[i][j] == '.':
            sg.append((i, j))
        else:
            memo[i][j] = True
ans = 0
for ss in sg:
    for g in sg:
        if ss == g:
            continue
        q = [ss]
        cnt = 0
        dist = deepcopy(memo)
        dist[ss[0]][ss[1]] = True
        while q:
            cnt += 1
            qq = []
            for y, x in q:
                for dy, dx in move:
                    xx, yy = x + dx, y + dy
                    if xx < 0 or yy > h - 1 or yy < 0 or xx > w - 1:
                        continue
                    if dist[yy][xx]:
                        continue
                    dist[yy][xx] = True
                    qq.append((yy, xx))
            q = qq
            if g in q:
                ans = max(ans, cnt)
                break
print(ans)


Fixed submission:
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
sys.setrecursionlimit(10 ** 7)

from copy import deepcopy

h, w = map(int, readline().split())
s = [input() for i in range(h)]
q = []
start = []
dist = [[False] * w for _ in range(h)]
for i in range(h):
    for j in range(w):
        if s[i][j] != '#':
            start.append((i, j))
        else:
            dist[i][j] = True
test = [(0, 1), (0, -1), (1, 0), (-1, 0)]
ans = []
while start:
    check = deepcopy(dist)
    q = [start.pop()]
    check[list(q)[0][0]][list(q)[0][1]] = True
    cnt = -1
    while q:
        cnt += 1
        qq = []
        memo = len(q)
        memo_cnt = 0
        for y, x in q:
            for dy, dx in test:
                yy, xx = y + dy, x + dx
                if xx < 0 or yy < 0 or xx > w - 1 or yy > h - 1 or check[yy][xx]:
                    memo_cnt += 1
                    continue
                else:
                    check[yy][xx] = True
                    qq.append((yy, xx))
        q = qq
    ans.append(cnt)
else:
    print(max(ans))

Buggy python submission (verdict: Time Limit Exceeded):
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
sys.setrecursionlimit(10 ** 7)

from copy import deepcopy

h, w = map(int, readline().split())
s = [readline().rstrip().decode() for _ in range(h)]
move = [(0, 1), (0, -1), (1, 0), (-1, 0)]
sg = []
memo = [[False] * w for _ in range(h)]
for i in range(h):
    for j in range(w):
        if s[i][j] == '.':
            sg.append((i, j))
        else:
            memo[i][j] = True
ans = 0
for ss in sg:
    for g in sg:
        q = [ss]
        cnt = 0
        dist = deepcopy(memo)
        dist[ss[0]][ss[1]] = True
        flag = False
        while q:
            cnt += 1
            qq = []
            for y, x in q:
                for dy, dx in move:
                    xx, yy = x + dx, y + dy
                    if xx < 0 or yy > h - 1 or yy < 0 or xx > w - 1:
                        continue
                    if dist[yy][xx]:
                        continue
                    dist[yy][xx] = True
                    if (yy, xx) == g:
                        flag = True
                        ans = max(ans, cnt)
                        break
                    qq.append((yy, xx))
            q = qq
print(ans)


Fixed submission:
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
sys.setrecursionlimit(10 ** 7)

from copy import deepcopy

h, w = map(int, readline().split())
s = [input() for i in range(h)]
q = []
start = []
dist = [[False] * w for _ in range(h)]
for i in range(h):
    for j in range(w):
        if s[i][j] != '#':
            start.append((i, j))
        else:
            dist[i][j] = True
test = [(0, 1), (0, -1), (1, 0), (-1, 0)]
ans = []
while start:
    check = deepcopy(dist)
    q = [start.pop()]
    check[list(q)[0][0]][list(q)[0][1]] = True
    cnt = -1
    while q:
        cnt += 1
        qq = []
        memo = len(q)
        memo_cnt = 0
        for y, x in q:
            for dy, dx in test:
                yy, xx = y + dy, x + dx
                if xx < 0 or yy < 0 or xx > w - 1 or yy > h - 1 or check[yy][xx]:
                    memo_cnt += 1
                    continue
                else:
                    check[yy][xx] = True
                    qq.append((yy, xx))
        q = qq
    ans.append(cnt)
else:
    print(max(ans))