If a Python module X is licensed under GPL and a Python script or module Y imports X, does it make Y necessarily GPL also?
Nothing makes Y GPL, short of the author of Y licensing it under GPL or a court order requiring them to do so.
Using a GPL code in Y without licensing Y under GPL makes such use illegal. This only affect the author of Y if they explicitly enable such use, e.g. they distribute Y alongside with X, or Y is distributed alone while X is a well-known GPL library one could expect to be installed on a target system. This would not turn Y license into GPL automatically, but it would open the author of Y to lawsuits, and releasing Y under GPL could be one of the possible outcomes. Alternatively, the author of Y could chose to pay damages to their users, retract the non-compliant version of Y and refrain from distributing Y until it is modified in a way to make it compliant.
If Y is distributed with a closed-sourced version of X, there will be no violation even if an open-source version of X exists and users could technically switch to it by altering Y.