Change an instance of default_features in Cargo.toml to default-features

[LikeLakers2]

As per cargo itself when running a cargo build:

default_features is deprecated in favor of default-features and will not work in the 2024 edition (in the quickcheck dependency)

Since default_features and default-features work the same, this should not change any functionality beyond removing the warning when building itertools. To be sure, however, I ran cargo test after the change, and all tests passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.35%. Comparing base (6814180) to head (3d7037e).
Report is 127 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #985      +/-   ##
==========================================
- Coverage   94.38%   94.35%   -0.04%     
==========================================
  Files          48       49       +1     
  Lines        6665     6768     +103     
==========================================
+ Hits         6291     6386      +95     
- Misses        374      382       +8     

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[jswrenn]

Thanks!