Issue #2937542 by alexpott, mikelutz, larowlan, dawehner, Mile23, catch, Gábor...

Issue #2937542 by alexpott, mikelutz, larowlan, dawehner, Mile23, catch, Gábor Hojtsy, Chi: Not setting the strict option of the Choice constraint to true is deprecated since Symfony 3.4 and will throw an exception in 4.0

(cherry picked from commit c9dd6558)