Don't use deprecated method in impl Arbitrary for DateTime

[pitdicker]

Missed in #1175. I also modified the CI to check the arbitrary feature.

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (ec557d2) 91.72% compared to head (bdd6148) 91.72%.

Files	Patch %	Lines
src/month.rs	0.00%	2 Missing ⚠️
src/datetime/mod.rs	0.00%	1 Missing ⚠️
src/naive/datetime/mod.rs	0.00%	1 Missing ⚠️
src/offset/utc.rs	0.00%	1 Missing ⚠️
src/weekday.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1336   +/-   ##
=======================================
  Coverage   91.72%   91.72%           
=======================================
  Files          38       38           
  Lines       17267    17267           
=======================================
  Hits        15839    15839           
  Misses       1428     1428           

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[pitdicker]

It turns out arbitrary doesn't support no_std (see rust-fuzz/arbitrary#38, rust-fuzz/arbitrary#74).

Because there is a chance it may in the future, I thought it best to not make the arbitrary feature imply std, but to put our implementations behind an extra std feature gate.

[pitdicker]

Converted to draft for now. Maybe my comment in rust-fuzz/arbitrary#74 (comment) gets a reply.

[pitdicker]

This is a small PR that replaces the use of a deprecated method and sets up a CI test for the arbitrary feature. I don't think we need to wait any longer on seeing changes to arbitrary to merge this.