Add PHP 8.1 type hints for ArrayAccess, IteratorAggregate, and Countable

[neildaniels]

PHP 8.1 added type hints to the ArrayAccess, IteratorAggregate, and Countable interfaces. This PR matches to type hints to avoid signature mismatch errors.

[codecov-commenter]

Codecov Report

Merging #247 (3973ff1) into 4.0 (e330e8f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##                4.0     #247   +/-   ##
=========================================
  Coverage     84.45%   84.45%           
- Complexity     2166     2168    +2     
=========================================
  Files           196      196           
  Lines          5454     5454           
=========================================
  Hits           4606     4606           
  Misses          848      848           

Impacted Files	Coverage Δ
lib/Tmdb/Model/Common/GenericCollection.php	74.28% <100.00%> (+0.24%)	⬆️
lib/Tmdb/Client.php	90.58% <0.00%> (-0.11%)	⬇️

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[neildaniels]

I tempered the type hint changes (namely, avoiding the use of mixed) to remain backwards compatible but still avoid throwing errors on PHP 8.1.

[wtfzdotnet]

Cheers @neildaniels , thanks! :-)