Having labgrid installed without extras:

$ pip freeze | grep pyserial
pyserial-labgrid==3.5.0.2
$ python -c 'import serial; print(serial.VERSION)'
3.5.0.2

With modbusrtu or dev extra (pulling in minimalmodbus, which depends on pyserial):

$ pip freeze | grep pyserial
pyserial==3.5
pyserial-labgrid==3.5.0.2
$ python -c 'import serial; print(serial.VERSION)'
3.5

This means pyserial effectively overwrites pyserial-labgrid.

I don't know if pyserial always overwrites pyserial-labgrid, it probably depends on pip's dependency resolving strategy.