Fix performance in get_imports regexp

[AlekseyLobanov]

What does this PR do?

Just found that one of the Regular Expressions may be very slow (O(N^3)) on some special inputs.

Something like 'try:' + ' ' * 1500 may take a lot of time. I replaced it with the same logic but faster

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

@gante

[AlekseyLobanov]

It is not a typo but still a small change with existing tests