Fix #836, fix #760, fix #200

I tried python 3.8 and 3.9, I let install the latest panda version (v1.2.3, .. happen only once ^^) and all the tests ran smoothly
Except one in TestACElectricityBus.test_benchmark_AB_grid_pv where it is a type mismatch which leads the two timeseries to not equal (one is int64, whereas the other float64) --> easily fixable. I would therefore suggest we use the latest panda version again.

Changes proposed in this pull request:

• Forbid pyomo v 5.7.3
• add plotly to the default to generate pngs

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)

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