php 7.3 support

[marchesesimone]

hello I report that some dependencies of the library are not compatible with php 7.3.
Below are the errors found:

`FILE: sites/all/libraries/mailchimp/vendor/phpunit/phpunit-mock-objects/tests/_fixture/SingletonClass.php

FOUND 1 ERROR AFFECTING 1 LINE
17 | ERROR | Visibility for magic method __sleep must be public. Found: private
| | (PHPCompatibility.FunctionDeclarations.NonStaticMagicMethods.__sleepMethodVisibility)

FILE: /sites/all/libraries/mailchimp/vendor/sebastian/diff/tests/ParserTest.php

FOUND 3 ERRORS AFFECTING 3 LINES
83 | ERROR | The body of a heredoc/nowdoc can not contain the heredoc/nowdoc closing marker as text at the start of a line since PHP 7.3.
| | (PHPCompatibility.Syntax.NewFlexibleHeredocNowdoc.ClosingMarkerNoNewLine)
126 | ERROR | The body of a heredoc/nowdoc can not contain the heredoc/nowdoc closing marker as text at the start of a line since PHP 7.3.
| | (PHPCompatibility.Syntax.NewFlexibleHeredocNowdoc.ClosingMarkerNoNewLine)
134 | ERROR | The body of a heredoc/nowdoc can not contain the heredoc/nowdoc closing marker as text at the start of a line since PHP 7.3.
| | (PHPCompatibility.Syntax.NewFlexibleHeredocNowdoc.ClosingMarkerNoNewLine)

Time: 13 mins, 31.46 secs; Memory: 204.01MB
`

[mariacha]

Since those errors are in items pulled into the vendor folder, this is only relevant if you're running unit tests on the api itself. For phpunit/phpunit-mock-objects the project is archived, so probably the next step is to find a replacement generally. For sebastian/diff that's a test that would only be run during unit testing for that package, not for mailchimp. Unless I'm missing something, this seems like it's not blocking any of the functionality of the Mailchimp API module itself, so the fix feels pretty low priority.