Update method signatures to match with interfaces

v1.4.0

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First release with support for PHP 8.0.
Internally, all PHPUnit tests are updated to use `self::` syntax instead of `$this->` for assertions.
Code is reformatted to use 4 spaces as well.

v1.2.0

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 - Dropped Travis CI in favor of GitHub Actions CI
 - Dropped support for PHP 7.2 and older.
 - Using PHPUnit ^9.2
 - Improvements in tests
 - Added return and parameter types to several of the methods.

v1.1.4

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Adds a .gitattributes file to ignore certain unnecessary files in dist

v1.1.2

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This version contains a minor fix for PHP 7.4 compatibility with bool array-index deprecation notice.

v1.1.1

* Add automated tests for PHP 7.3
* Do not ignore test failures on PHP nighty builds.
* Fix the PHP requirement badge in README.me.

Add return type hints for Strict::offsetExists() method

Fix Strict::__construct pParameter docblock

Closes #2