Fix/python bindings python requirements

• Add the requirement Python >= 3.6 for the Python bindings (#275). If the version is less, a hopefully informative message, friendly, but still attention-grabbing message is printed.
• Remove the check for the Python interpreter which was unreachable anyway due to a check further up
• Fixes #297 (closed)
• Unconditionally define add_python_targets for ease of use

This MR only concerns the Python bindings not Python support for possible other usages. This is why I didn't require the Python version via the find_package mechanism for now.

added buildsystem python labels

assigned to @andreas.dedner

requested review from @simon.praetorius

changed the description

added 1 commit

• fe4d12c6 - [python][cmake] Add status message that the Python bindings have been enabled

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@tkoch looks good to me. Did we decide to add changes to the requirements to the changlog now? At least I think having an entry there that Python 3.6 is required for bindings would be a good idea.

added 1 commit

• 7ecc1fbb - [changelog] Mention required Python version for bindings

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resolved all threads

mentioned in merge request !1102 (closed)

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• 7ecc1fbb...5d96cbcc - 4 commits from branch master
• 67cb9c77 - [python][cmake] Require Python version 3.6 for the bindings (only) and improve user messages
• 9e911c03 - [python][cmake] Remove unreachable check for Python interpreter
• 54a1be67 - [python][cmake] Add status message that the Python bindings have been enabled
• 8ba63fad - Set DUNE_ENABLE_PYTHONBINDINGS to OFF if there is no python.
• a9b3675e - [cmake][python] Harmonize messages

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added 1 commit

• 260255a7 - [cmake][python] Harmonize messages

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added 1 commit

• a2d2b353 - [cmake][python] Harmonize messages

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added 1 commit

• 73169abb - Unconditionally define add_python_targets for ease of use.

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added 2 commits

• 44451b0c - [cmake][python] Properly introduce and overwrite the global option DUNE_ENABLE_PYTHONBINDINGS
• fa46869e - [cmake] Add todo comment for add_python_targets rename

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