Ignore python bytecode

Since the prebuilt contains python source, executing it will produce
bytecode files. Ignore these in git to avoid the repo becoming dirty after
a build.

Test: Run Trusty build (uses python libraries), prebuilt no longer dirty
Change-Id: I9161c99be87c45dfd3471fbdcaf340e6a255aa3b
(cherry picked from commit be518ef4272ce61fd6dad9db73e9be74853b3974)
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..d4acfbf
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,3 @@
+# Ignore python temporary files
+__pycache__/
+*.py[cod]