It is not compatible with oemof

Changes proposed in this pull request:

• Prevent pyomo to install version 6.0 in the requirements.txt

The following steps were realized, as well (if applies):

• Use in-line comments to explain your code
• Write docstrings to your code (example docstring)
• For new functionalities: Explain in readthedocs
• Write test(s) for your new patch of code (pytests, assertion debug messages)
• Update the CHANGELOG.md
• Apply black (black . --exclude docs/)
• Check if benchmark tests pass locally (EXECUTE_TESTS_ON=master pytest)

_{For more information on how to contribute check the CONTRIBUTING.md.}