TST: remove IPython warning in debug session

[tupui]

What does this implement/fix?

When starting a debug session in PyCharm, the console cannot be used due to IPython warnings being caught.

Traceback (most recent call last):
  File "/Applications/PyCharm.app/Contents/plugins/python/helpers/pydev/_pydevd_bundle/pydevd_console_integration.py", line 84, in get_code_executor
    code_executor = getattr(__builtin__, 'interpreter')
AttributeError: module 'builtins' has no attribute 'interpreter'
During handling of the above exception, another exception occurred:
...
DeprecationWarning: The `use_readline` parameter is deprecated and ignored since IPython 6.0.

The fix consist in skipping the warning in the pytest configuration. Basically it's the first thing I do when I create a new branch to work on.

Additional information

This is also done here: mne-tools/mne-python#8524

[rgommers]

LGTM, thanks @tupui

[tupui]

Great thanks!