Drop dependency on ruby2_keywords

[Earlopain]

Now that Ruby < 2.7 is not supported (#345) this isn't needed anymore

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.32%. Comparing base (729a91d) to head (e0368b9).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #578 +/- ## ========================================== - Coverage 94.34% 94.32% -0.03% ========================================== Files 17 17 Lines 460 458 -2 ========================================== - Hits 434 432 -2 Misses 26 26 ```

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[Earlopain]

Hi, the dependency is not needed anymore because ruby2_keyword methods are defined by ruby natively from 2.7 and up. It's only necesseary for downward compatibility.

$ rbenv local 2.6.10
$ irb
irb(main):001:0> RUBY_VERSION
=> "2.6.10"
irb(main):002:0> Module.respond_to?(:ruby2_keywords, true)
=> false

$ rbenv local 2.7.8
$ irb
irb(main):001:0> RUBY_VERSION
=> "2.7.8"
irb(main):002:0> Module.respond_to?(:ruby2_keywords, true)
=> true