Tests are part of the production code

Hello everyone!

I noticed a few days ago the following:

```

 ~/tmp $ pip freeze

 ~/tmp $ pip install python-engineio

Collecting python-engineio

  Downloading https://files.pythonhosted.org/packages/9c/d3/de0fa7ebebd054de308e6ac397bf9e11ea42924795a38005a21ea001b114/python_engineio-3.8.1-py2.py3-none-any.whl (119kB)

    100% |████████████████████████████████| 122kB 6.3MB/s 

Collecting six>=1.9.0 (from python-engineio)

  Using cached https://files.pythonhosted.org/packages/73/fb/00a976f728d0d1fecfe898238ce23f502a721c0ac0ecfedb80e0d88c64e9/six-1.12.0-py2.py3-none-any.whl

Installing collected packages: six, python-engineio

Successfully installed python-engineio-3.8.1 six-1.12.0

You are using pip version 10.0.1, however version 19.1.1 is available.

You should consider upgrading via the 'pip install --upgrade pip' command.

 ~/tmp $ pip freeze

python-engineio==3.8.1

six==1.12.0

You are using pip version 10.0.1, however version 19.1.1 is available.

You should consider upgrading via the 'pip install --upgrade pip' command.

 ~/tmp $ python

Python 3.7.1 (default, Nov 15 2018, 09:36:57) 

[GCC 8.2.1 20181105 (Red Hat 8.2.1-5)] on linux

Type "help", "copyright", "credits" or "license" for more information.

>>> import tests

>>> 

 ~/tmp $ pip uninstall python-engineio

Uninstalling python-engineio-3.8.1:

  Would remove:

    /home/pablo.verges/.pyenv/versions/3.7.1/envs/tmp/lib/python3.7/site-packages/engineio/*

    /home/pablo.verges/.pyenv/versions/3.7.1/envs/tmp/lib/python3.7/site-packages/python_engineio-3.8.1.dist-info/*

    /home/pablo.verges/.pyenv/versions/3.7.1/envs/tmp/lib/python3.7/site-packages/tests/*

Proceed (y/n)? y

  Successfully uninstalled python-engineio-3.8.1

You are using pip version 10.0.1, however version 19.1.1 is available.

You should consider upgrading via the 'pip install --upgrade pip' command.

 ~/tmp $ python

Python 3.7.1 (default, Nov 15 2018, 09:36:57) 

[GCC 8.2.1 20181105 (Red Hat 8.2.1-5)] on linux

Type "help", "copyright", "credits" or "license" for more information.

>>> import tests

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

ModuleNotFoundError: No module named 'tests'

```



In other words: `python-engineio` install a `tests` package into my `site-packages`... and I kind of don't like this (and I guess that this is unexpected behavior).

[]