Pycharm debugger for apache_beam/*_test.py broken

This currently affects pipeline_test.py and pvalue_test.py.
It seems that "import io" is interpreted as importing apache_beam.io, which fails.

In Python 2.7 the stacktrace shows:
```

Testing started at 3:48 PM ...
/usr/local/google/home/ehudm/virtualenvs/beamenv/bin/python /usr/local/google/home/ehudm/.local/share/JetBrains/Toolbox/apps/PyCharm-C/ch-0/191.7479.30/helpers/pydev/pydevd.py
--multiproc --qt-support=auto --client 127.0.0.1 --port 41493 --file /usr/local/google/home/ehudm/.local/share/JetBrains/Toolbox/apps/PyCharm-C/ch-0/191.7479.30/helpers/pycharm/_jb_nosetest_runner.py
--path /usr/local/google/home/ehudm/src/beam/sdks/python/apache_beam/pvalue_test.py
Traceback (most
recent call last):
File "/usr/local/google/home/ehudm/.local/share/JetBrains/Toolbox/apps/PyCharm-C/ch-0/191.7479.30/helpers/pydev/pydevd.py",
line 15, in
from _pydevd_bundle.pydevd_constants import IS_JYTH_LESS25, IS_PY34_OR_GREATER,
IS_PY36_OR_GREATER, IS_PYCHARM, get_thread_id, \
File "/usr/local/google/home/ehudm/.local/share/JetBrains/Toolbox/apps/PyCharm-C/ch-0/191.7479.30/helpers/pydev/_pydevd_bundle/pydevd_constants.py",
line 169, in
from _pydev_imps._pydev_saved_modules import thread
File "/usr/local/google/home/ehudm/.local/share/JetBrains/Toolbox/apps/PyCharm-C/ch-0/191.7479.30/helpers/pydev/_pydev_imps/_pydev_saved_modules.py",
line 15, in
import xmlrpclib
File "/usr/lib/python2.7/xmlrpclib.py", line 145, in

import httplib
File "/usr/lib/python2.7/httplib.py", line 80, in
import mimetools

File "/usr/lib/python2.7/mimetools.py", line 6, in
import tempfile
File "/usr/lib/python2.7/tempfile.py",
line 32, in
import io as _io
File "/usr/local/google/home/ehudm/src/beam/sdks/python/apache_beam/io/__init__.py",
line 22, in
from apache_beam.io.avroio import *
File "/usr/local/google/home/ehudm/src/beam/sdks/python/apache_beam/__init__.py",
line 97, in
from apache_beam import coders
File "/usr/local/google/home/ehudm/src/beam/sdks/python/apache_beam/coders/__init__.py",
line 19, in
from apache_beam.coders.coders import *
File "/usr/local/google/home/ehudm/src/beam/sdks/python/apache_beam/coders/coders.py",
line 27, in
from builtins import object
File "/usr/local/google/home/ehudm/virtualenvs/beamenv/local/lib/python2.7/site-packages/builtins/__init__.py",
line 8, in
from future.builtins import *
File "/usr/local/google/home/ehudm/virtualenvs/beamenv/local/lib/python2.7/site-packages/future/builtins/__init__.py",
line 13, in
from future.builtins.misc import (ascii, chr, hex, input, isinstance, next,

File "/usr/local/google/home/ehudm/virtualenvs/beamenv/local/lib/python2.7/site-packages/future/builtins/misc.py",
line 43, in
from io import open
ImportError: cannot import name open

```

Imported from Jira [BEAM-7971](https://issues.apache.org/jira/browse/BEAM-7971). Original Jira may contain additional context.
Reported by: udim.