This is the basic schematic diagram with D1 going to the data line of the servo (the data line is typically the white wire coming from the servo). Your going to want to use an external power source for the servo power, typically between 4.8 - 6.0 Volts. While you could technically power the servo from the USB power if your servo is trying to move under load it could potentially exceed the max USB current and cause a temporary loss of power to the pyMCU as the resettable fuse gets triggered. So create a common ground between the pyMCU and the external power and then connect VCC from the servo only to the external power source and DO NOT connet VCC from pyMCU to the external power supply VCC.