Upgrade to new PHP, Symfony, PHPUnit + use DateTimeImmutable

[enumag]

[enumag]

@jakubzapletal I need to parse some of the formats in a new project. Instead of inventing a wheel I thought I'd use an existing library but it's a bit outdated. Are you still using it? If not, could I ask for write access so that I can maintain it from now on? Or should I fork the repository instead and create a duplicate on packagist?

[enumag]

If anyone else needs this I have a fork with these changes available as enumag/bank-statements.