Update requirements.txt for serial connection

Although pymavlink supports the use of serial connections with a flight control board and utilizes a serial connection package, this information is not reflected in the requirements.txt file.

Simply running "pip install -r pymavlink/requirements.txt" and attempting to use the mavutil module over serial will not work as expected.

Author: sethome2

requirements.txt

@@ -2,6 +2,7 @@ lxml>=3.6.0
 future>=0.15.2
 wheel>=0.37.1
 setuptools>=42
+pyserial>=3.5
 fastcrc
 
 # dev dependencies: