ERP5TypeTestCase: support using unpinDateTime as a context manager

exiting the context manager restores the original DateTime behavior
2 jobs for feat/pinDateTime-contextmanager in 0 seconds (queued for 1 second)
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-TestRunner2

01:59:33

passed ERP5.UnitTest-TestRunner2

01:52:29