Update minimum PHP version to 7.3

[jonyo]

Update the required minimum PHP version: 5.* has been EOL for a while now, and even 7.2 is about to be EOL soon. Make the minimum PHP version 7.3. As part of it:

• Update PHPUnit to version 9 for compatibility, and update tests for the new version.
• Add type declarations and strict typing to all methods.
• Since this will make it not backwards compatible (if library is used on older version of PHP), will need to target a new major release.