Fix eager import of cftime in _maybe_cast_to_cftimeindex

[peterhron]

Description

Checklist

• Closes [PERFORMANCE] object indexes repeatedly attempt to import missing cftime (regression) #11559
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

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Fixes a regression with eagerly-importing cftime, leading to performance degradation.

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[Illviljan]

Could you add a test to make sure we dont regress again?

[peterhron]

Could you add a test to make sure we dont regress again?

I have added a simple test in test_indexes. Please have a glance, if you will. Thanks.

[peterhron]

@dcherian thanks for the quick approve, i just added a test and rebased today. Should be good to go, otherwise :)