[bug] Missing dependency pytest

Issue Report Checklist

[X] Searched the issues page for similar reports
[X] Read the relevant sections of the documentation
[X] Browse the tutorials and tests for usefull code snippets and examples of use
[X] Reproduced the issue after updating with pip install --upgrade pandapower (or git pull)
[X] Tried basic troubleshooting (if a bug/error)
- [X] Restarted interpreter
- [X] Checked the pythonpath
[X] Completed the Problem Description, What steps reproduce the problem and Versions sections below

Problem Description

Related (but not the same) to #1985 I also found out a missing dependency for pandapower version 2.12 (still the CI of some of my projects). This time it's not related to a python version, but the "pytest" package is missing (in the setup.py / setup.cfg) and prevents the use of pandapower when it's not installed.

This problem did not appear with previous version of pandapower.

What steps reproduce the problem?

Run the following script:

python3 -m virtualenv venv_test_pp
source venv_test_pp/bin/activate
pip install --upgrade pandapower
python -c "import pandapower as pp"

What is the expected output? What do you see instead?

This results in the error:

matplotlib could not be imported. It is required for many plotting functions. 
Thus, this might lead to errors with some plotting functions. To install all pandapower dependencies, pip install pandapower['all'] can be used.
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "venv_test_pp/lib/python3.8/site-packages/pandapower/__init__.py", line 35, in <module>
    import pandapower.test
  File "venv_test_pp/lib/python3.8/site-packages/pandapower/test/__init__.py", line 7, in <module>
    from pandapower.test.conftest import *
  File "venv_test_pp/lib/python3.8/site-packages/pandapower/test/conftest.py", line 8, in <module>
    import pytest
ModuleNotFoundError: No module named 'pytest'

Paste Traceback/Error Below (if applicable)


Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "venv_test_pp/lib/python3.8/site-packages/pandapower/__init__.py", line 35, in <module>
    import pandapower.test
  File "venv_test_pp/lib/python3.8/site-packages/pandapower/test/__init__.py", line 7, in <module>
    from pandapower.test.conftest import *
  File "venv_test_pp/lib/python3.8/site-packages/pandapower/test/conftest.py", line 8, in <module>
    import pytest
ModuleNotFoundError: No module named 'pytest'

Versions

pandas version:
networkx version:
scipy version:
numpy version:
packaging version:
tqdm version:
deepdiff version:
Operating System name/version: ubuntu 20.04

It does not depends on any of these packages, the bug is a missing package in setup.py / setup.cfg or when importing pandapower (in __init__.py line 35) :

try:
    import pandapower.test
except ImportError:
    # pytest is not installed, the "test" module will not be available
    pass

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e2nIEE / pandapower