PHP 8.1 compatibility

[courtney-miles]

Overview

Issue: #70

Updates package for PHP 8.1 compatibility.

• Implementations of the methods for the \Iterator interface have had #[\ReturnTypeWillChange] added, since these methods have a return type as a PHP 8.1
• strptime has been deprecated. StrptimeFormatTransformer has been introduced to convert a pattern from Python format to the format understood by date_parse_from_format. (I.e. %Y-%m-%d is changed to Y-m-d.)

Dev changes:

• PHP CS Fixer has had it's version fixed to 3.4.0 as this is the last version with compatibility with PHP 7.2.
• PHPUnit is pinned to versions between 7.5 and 10.0.
• Test data provider methods have changed to static for compatibility with the next version of PHPUnit

[OriHoch]

nice work!

the strptime change seems the most risky but I reviewed it and the code looks fine, also I guess it's fairly simple conversion of formats and the date/time tests pass so I guess it's good
👍