Python serial between Arduino and Raspberry Pi: Data is modified upon reception
The arduino is not well suited to doing floating point math, nor is it partiularly well suited to doing string manipulation.

You would be better off by sending the value read from the analog input directly to the python code on the Pi and do the math and string manipulation in Python.

On the arduino side just do something like this:

int value;

value = analogRead(A0); // reads a 12bit integer value from the Analog input
Serial.println(value);  // converts the integer to a string and sends it over the serial port


Then on the Pi side:

str = ser.readline()  # read a string from the serial port

value = float(str)    # convert a string to a floating point number

volt = 5.0 * value / 1023.0  # compute the voltage