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/tools/bazel.rc b/tools/bazel.rc
index a0dd7a2..1dee9b5 100644
--- a/tools/bazel.rc
+++ b/tools/bazel.rc
@@ -9,6 +9,9 @@
 # Use the malloc we want.
 build --custom_malloc=//tools/cpp:malloc
 
+# Use our hermetic Python runtime.
+build --python_top=//tools/python:runtime
+
 build:asan --copt -fsanitize=address
 build:asan --linkopt -fsanitize=address --linkopt -ldl
 build:asan --platform_suffix=-asan