Use non-deprecated option with days

[chanhosuh]

Description

Numpy no longer allows the "Y" unit in its timedelta constructor, e.g. timedelta64(1, 'Y'), due to a "year" being an ambiguous number of nanoseconds.

We fix this by using timedelta64(365, 'D'), which does not materially impact our annualization calculation.

Fixes #233.

Hygiene checklist

• [ ] Changelog entry
• [ ] Everything public has a Numpy-style docstring (modules, public functions, classes, and public methods)
• [x] Commit history is cleaned-up with minor changes squashed together and descriptive commit messages following Tim Pope's style

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Coverage report

The coverage rate went from 74.93% to 74.93% :arrow_down: The branch rate is 61%.

100% of new lines are covered.

Diff Coverage details (click to unfold)

### curvesim/metrics/metrics.py `100%` of new lines are covered (`83.76%` of the complete file).