Update symfony/process from v3.4.28 to v3.4.31

[violinist-bot]

If you have a high test coverage index, and your tests for this pull request are passing, it should be both safe and recommended to merge this update.

Here is a list of changes between the version you use, and the version this pull request updates to:

• d822cb6 [Process] fix return type declarations
• 10f4eff [HttpFoundation] fix return type declarations
• 5179b12 Fix inconsistent return points.
• 60838d6 [Process] Doc block backport.
• def2e16 Fix some docblocks.
• 8f99ff4 Remove superfluous phpdoc tags
• 910b30e Fix return statements
• dcd467d minor #32800 Improve some URLs (Arman-Hosseini)
• cddcf9b Improve some URLs
• 348bfd4 Use assertStringContainsString when needed
• 9708054 Remove use of ForwardCompatTrait
• 294250e Fix deprecated phpunit annotation
• f1b06cb Replace calls to setExpectedException by Pollyfill
• df06f4e Fix assertInternalType deprecation in phpunit 9
• c95d971 Make tests support phpunit 8
• d129c01 Use willReturn() instead of will(returnValue()).

---

This is an automated pull request from Violinist: Continuously and automatically monitor and update your composer dependencies. Have ideas on how to improve this message? All violinist messages are open-source, and can be improved here.