text: Title: MPI compatibility
  Body: - [x] I have visited the [source website], and in particular read the [known issues]
  - [x] I have searched through the [issue tracker] for duplicates
  - [x] I have mentioned version numbers, operating system and environment, where applicable:
    ```python
    import tqdm, sys
    print(tqdm.__version__, sys.version, sys.platform)
    ```
  
  ```
  ('4.31.1', '2.7.8 (default_cci, Sep  6 2016, 11:26:30) \n[GCC 4.4.7 20120313 (Red Hat 4.4.7-16)]', 'linux2')
  ```
  
  I noticed that `mpi4py` complains if I import `tqdm` after `mpi4py`. This happens with OpenMPI 2.0.0, 2.1.6 and 3.0.3 but not with 4.0.0. OpenMPI is not happy with `fork()` system calls. However, `strace` does not indicate `tqdm` is calling `fork()` or `system()`.
  
  Test code:
  ```python
  from mpi4py import MPI
  print 'MPI imported'
  print MPI.COMM_WORLD.Get_rank()
  
  from tqdm import tqdm
  print 'tqdm imported'
  ```
  
  ```
  $ mpirun -n 2 python ~/scripts/mpitest.py
  MPI imported
  0
  MPI imported
  1
  --------------------------------------------------------------------------
  A process has executed an operation involving a call to the
  "fork()" system call to create a child process.  Open MPI is currently
  operating in a condition that could result in memory corruption or
  other system errors; your job may hang, crash, or produce silent
  data corruption.  The use of fork() (or system() or other calls that
  create child processes) is strongly discouraged.
  
  The process that invoked fork was:
  
    Local host:          [[3456,1],0] (PID 54613)
  
  If you are *absolutely sure* that your application will successfully
  and correctly survive a call to fork(), you may disable this warning
  by setting the mpi_warn_on_fork MCA parameter to 0.
  --------------------------------------------------------------------------
  tqdm imported
  tqdm imported
  ```
  
    [source website]: https://github.com/tqdm/tqdm/
    [known issues]: https://github.com/tqdm/tqdm/#faq-and-known-issues
    [issue tracker]: https://github.com/tqdm/tqdm/issues?q=
  