tqdm.write under multiprocessing.Pool make bars overlapped

[yihengli]

• I have visited the source website, and in particular
  read the known issues
• I have searched through the issue tracker for duplicates
• I have mentioned version numbers, operating system and
  environment, where applicable:

  import tqdm, sys
  print(tqdm.__version__, sys.version, sys.platform)

---

Env

4.26.0 3.7.2 (default, Dec 29 2018, 00:00:04)
[Clang 4.0.1 (tags/RELEASE_401/final)] darwin . (MacOS . 10.13.4)

Reproduce

from time import sleep
from tqdm import tqdm
from multiprocessing import Pool


def foo(a):
    tqdm.write("Working on %d" % a)
    sleep(0.5)
    return a

if __name__ == '__main__':
    with Pool(2) as p:
        list(tqdm(p.imap(foo, range(20)), total=20))

This might be relevant to #407