python: Add in pyserial to the list of requirements

This isn't necessary for running things in simulation, but
when it comes to running robots on FPGA, we need this.

Change-Id: If38537d99d558064e2c0d62e70638b452984581f
diff --git a/python-requirements.txt b/python-requirements.txt
index d15022f..a4b7816 100644
--- a/python-requirements.txt
+++ b/python-requirements.txt
@@ -17,6 +17,9 @@
 sortedcontainers
 hypothesis
 
+# Robot test framework for FPGA tests
+pyserial
+
 # camkes-deps
 
 hjson