dependent python-concurrent.futures not installed

[Susumu Miwa]

I install python-apscheduler.

but dependent python-concurrent.futures not installed.

[environment]
$ lsb_release -rd
Description: Ubuntu 18.04.3 LTS
Release: 18.04

$ apt-cache policy python-apscheduler
python-apscheduler:
  Installed: 3.4.0-2ubuntu1
  Candidate: 3.4.0-2ubuntu1
  Version table:
 *** 3.4.0-2ubuntu1 500
        500 http://jp.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        500 http://jp.archive.ubuntu.com/ubuntu bionic/universe i386 Packages
        100 /var/lib/dpkg/status

$ python --version
Python 2.7.17

[procedure]
install python-apscheduler

$ sudo apt install python-apscheduler

Prepare the following script

$ cat a.py
from apscheduler.schedulers.blocking import BlockingScheduler
from apscheduler.executors.pool import ThreadPoolExecutor

if __name__ == '__main__':
  print "hello"

If I execute it, an error will occur.

$ python a.py
Traceback (most recent call last):
  File "a.py", line 1, in <module>
    from apscheduler.schedulers.blocking import BlockingScheduler
  File "/usr/lib/python2.7/dist-packages/apscheduler/schedulers/blocking.py", line 5, in <module>
    from apscheduler.schedulers.base import BaseScheduler, STATE_STOPPED
  File "/usr/lib/python2.7/dist-packages/apscheduler/schedulers/base.py", line 17, in <module>
    from apscheduler.executors.pool import ThreadPoolExecutor
  File "/usr/lib/python2.7/dist-packages/apscheduler/executors/pool.py", line 2, in <module>
    import concurrent.futures
ImportError: No module named concurrent.futures

[examined]

python-concurrent.futures not installed.

$ apt list --installed python-apscheduler python-concurrent.futures
Listing... Done
python-apscheduler/bionic,bionic,now 3.4.0-2ubuntu1 all [installed]

depends don't show python-concurrent.futures?

$ apt depends python-apscheduler
python-apscheduler
  Depends: python-pkg-resources
  Depends: python-six (>= 1.4.0)
  Depends: python-tz
  Depends: python-tzlocal
  Depends: <python:any> (<< 2.8)
    python:i386
    python
  Depends: <python:any> (>= 2.7.5-5~)
    python:i386
    python

written Build-Depends: python-concurrent.futures in debian/control.

https://packages.ubuntu.com/ja/bionic/python-apscheduler

I get apscheduler_3.4.0-2ubuntu1.debian.tar.xz .

see debian/control

Source: apscheduler
Section: python
Priority: optional
Maintainer: Ubuntu Developers <email address hidden>
XSBC-Original-Maintainer: Laszlo Boszormenyi (GCS) <email address hidden>
Build-Depends: debhelper (>= 11), dh-python,
 python-all (>= 2.7),
 python-setuptools, python-setuptools-scm,
 python-concurrent.futures, python-tzlocal, python-six, python-pytest,
 ^^^^^^^^^^^^^^^^^^^^^^^^^^

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