[MNT, BUG] Modify njit func generator to support python functions

[baraline]

Reference Issues/PRs

Fixes #1020

What does this implement/fix? Explain your changes.

Modify generate_new_default_njit_func function to support python function and be coherent when DISABLE_JIT option is set to True, so it returns a python function.

Does your contribution introduce a new dependency? If yes, which one?

No.

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I have added the following labels to this PR based on the title: [ $\color{#d73a4a}{\textsf{bug}}$, $\color{#EC843A}{\textsf{maintenance}}$ ].

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[TonyBagnall]

I've added the full pytest tag so it will run all the tests, not actually sure if setting this and/or codecov then runs without njit, @MatthewMiddlehurst?

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[MatthewMiddlehurst]

The tags added should be fine. Good work, This should fix the periodic tests and I think #1043 should make the future ones a lot more stable.