Add php 8 support

Merge pull request #15 from GuilhemN/php8

Add php8 support

v0.4.1: Merge pull request #12 from phansys/abstract_do_merge

Make `AbstractModel::doMerge()` abstract instead of call a nonexistent method `parse()`

v0.4.0: Merge pull request #5 from dragosprotung/items_properties

Items now extends from Schema (to have the same api than zircote/swagger-php).

v0.3.2: Merge pull request #3 from betzholz/master

Add Description to Item

v0.3.1

Merge pull request #4 from choonge/feature/remove-constructor-typehin…

v0.3.0: Merge pull request #2 from Prezent/parameter-ref

Allow parameter references

v0.2.4: Merge pull request #1 from GuilhemN/security

Add SecurityDefinitions support

v0.2.3

Allow using \stdClass

v0.2.2

Fix \ArrayAccess support

v0.2.1

Minor fix