PHP 8.0 compatibility, many thanks to @jrf_nl!
Switch from PHP CS Fixer to PHP CodeSniffer for coding standards
Create class constants for the debug levels in the POP3 class
Improve French, Slovenian, and Ukrainian translations
Improve file upload examples so file extensions are retained
Resolve PHP 8 line break issues due to a very old PHP bug being fixed
Avoid warnings when using old openssl functions
Improve Travis-CI build configuration