Use a (mostly) hermetic Python interpreter

It still requires python to be installed on the host to run the wrapper
scripts, but it's close to being fully hermetic.

This also requires/enables the following changes, unfortunately all at
the same time:
 * Use a downloaded f2py
 * Use a downloaded scipy/numpy/matplotlib
 * Fix a few things that don't run with the python version in their #!
 * Stop using bazel-generated __init__.py files, because those interfere
   with importing matplotlib nicely

Change-Id: Ife280464613d67cece9587b7d947f0b1d5466d7e
diff --git a/motors/BUILD b/motors/BUILD
index 395765e..a1375f4 100644
--- a/motors/BUILD
+++ b/motors/BUILD
@@ -148,3 +148,9 @@
     name = "simple_receiver",
     restricted_to = mcu_cpus,
 )
+
+py_library(
+    name = "python_init",
+    srcs = ["__init__.py"],
+    visibility = ["//visibility:public"],
+)