Use ABC base class

[phiwuu]

The main goal of this pull request is to resolve the pylint warning too-few-public-methods.

Please consider the following:

1. checkout main branch
2. remove too-few-public-methods from

   lobster/pylint3.cfg

   Line 8 in 79dfe7f

   too-few-public-methods,

3. call make lint and remember the output
4. checkout the branch internal/use-abc of this PR, and repeat steps 2 and 3

The errors in file htmldoc.py will disappear.

The following changes are necessary to make pylint happy:
The base class Menu_Item had a statement assert false, which is a code smell. In order to remove this, the class now inherits from ABC and the generate method is an abstract method.

Also removed those assert statements which verified only the type. At the beginning of the project these assertions had been used to develop a reliable implementation in the sense of ISO 26262, but the (ongoing) tool qualification efforts allow to follow Python's duck typing philosophy again.

[kedarnn]

Instead of assert here we can add a check and raise an error

[phiwuu]

Yes, that's true. But I wanted to keep the refactoring minimal. Raising an exception basically means a new code path, which should be tested by a unit test. There is another assertion of this kind: assert alignment in ("left", "right")
I recommend to do this in a separate pull request.

The main goal of this pull request is to resolve the pylint warning too-few-public-methods. Please do the following:

1. checkout main branch
2. remove too-few-public-methods from

   lobster/pylint3.cfg

   Line 8 in 79dfe7f

   too-few-public-methods,

3. call make lint, and remember the output
4. checkout the branch internal/use-abc of this PR, and repeat steps 2 and 3

The errors in file htmldoc.py will disappear. I should have made this clear from the beginning in the PR description. I will update it accordingly.

I removed the assert statements only as a clean-up side-task. Please approve the PR without further changes. Thank you!