Use a proper bazel toolchain for Python

This patch removes the old `--python_top` approach and instead turns
out Python toolchain into a proper Bazel toolchain. It's now used in
toolchain resolution.

    $ bazel run aos:flatbuffers_static --toolchain_resolution_debug
    WARNING: Option 'ui' is deprecated
    INFO: Invocation ID: 5ed4f6d5-e72d-4705-a853-c9a91264d74c
    INFO: Build option --toolchain_resolution_debug has changed, discarding analysis cache.
    INFO: ToolchainResolution:   Type @bazel_tools//tools/cpp:toolchain_type: target platform //tools/platforms:linux_x86: execution //tools/platforms:linux_x86: Selected toolchain //tools/cpp:cc-compiler-k8
    INFO: ToolchainResolution:   Type @bazel_tools//tools/python:toolchain_type: target platform //tools/platforms:linux_x86: execution //tools/platforms:linux_x86: Selected toolchain //tools/python:py_runtime
    INFO: ToolchainResolution:     Type @bazel_tools//tools/cpp:toolchain_type: target platform //tools/platforms:linux_x86: Rejected toolchain //tools/cpp:cc-compiler-armhf-debian; mismatching values: raspberry_pi
    INFO: ToolchainResolution:     Type @bazel_tools//tools/cpp:toolchain_type: target platform //tools/platforms:linux_x86: Rejected toolchain //tools/cpp:cc-compiler-roborio; mismatching values: roborio
    INFO: ToolchainResolution:     Type @bazel_tools//tools/cpp:toolchain_type: target platform //tools/platforms:linux_x86: Rejected toolchain //tools/cpp:cc-compiler-cortex-m4f; mismatching values: none, cortex_m4f
    INFO: ToolchainResolution:     Type @bazel_tools//tools/cpp:toolchain_type: target platform //tools/platforms:linux_x86: Rejected toolchain @local_config_cc//:cc-compiler-armeabi-v7a; mismatching values: arm, android
    INFO: ToolchainResolution: Target platform //tools/platforms:linux_x86: Selected execution platform //tools/platforms:linux_x86, type @bazel_tools//tools/python:toolchain_type -> toolchain //tools/python:py_runtime, type @bazel_tools//tools/cpp:toolchain_type -> toolchain //tools/cpp:cc-compiler-k8
    INFO: ToolchainResolution: Target platform //tools/platforms:linux_x86: Selected execution platform //tools/platforms:linux_x86,
    INFO: ToolchainResolution: Target platform //tools/platforms:linux_x86: Selected execution platform //tools/platforms:linux_x86,
    INFO: ToolchainResolution:   Type @bazel_tools//tools/cpp:toolchain_type: target platform //tools/platforms:linux_x86: execution //tools/platforms:linux_x86: Selected toolchain //tools/cpp:cc-compiler-k8
    INFO: ToolchainResolution:     Type @bazel_tools//tools/cpp:toolchain_type: target platform //tools/platforms:linux_x86: Rejected toolchain //tools/cpp:cc-compiler-armhf-debian; mismatching values: raspberry_pi
    INFO: ToolchainResolution:     Type @bazel_tools//tools/cpp:toolchain_type: target platform //tools/platforms:linux_x86: Rejected toolchain //tools/cpp:cc-compiler-roborio; mismatching values: roborio
    INFO: ToolchainResolution:     Type @bazel_tools//tools/cpp:toolchain_type: target platform //tools/platforms:linux_x86: Rejected toolchain //tools/cpp:cc-compiler-cortex-m4f; mismatching values: none, cortex_m4f
    INFO: ToolchainResolution:     Type @bazel_tools//tools/cpp:toolchain_type: target platform //tools/platforms:linux_x86: Rejected toolchain @local_config_cc//:cc-compiler-armeabi-v7a; mismatching values: arm, android
    INFO: ToolchainResolution: Target platform //tools/platforms:linux_x86: Selected execution platform //tools/platforms:linux_x86, type @bazel_tools//tools/cpp:toolchain_type -> toolchain //tools/cpp:cc-compiler-k8
    INFO: ToolchainResolution: Target platform //tools/platforms:linux_x86: Selected execution platform //tools/platforms:linux_x86,
    INFO: Analyzed target //aos:flatbuffers_static (0 packages loaded, 45236 targets configured).
    INFO: Found 1 target...
    INFO: Writing explanation of rebuilds to '/tmp/bazel_explain.log'
    Target //aos:flatbuffers_static up-to-date:
      bazel-bin/aos/flatbuffers_static
    INFO: Elapsed time: 2.740s, Critical Path: 0.07s
    INFO: 1 process: 1 internal.
    INFO: Build completed successfully, 1 total action
    INFO: Build completed successfully, 1 total action

Notice the selection of `//tools/python:py_runtime`.

Change-Id: I5adfae4cd11e73b5527a2fc98cef0de6a92464ab
Signed-off-by: Philipp Schrader <philipp.schrader@gmail.com>
diff --git a/WORKSPACE b/WORKSPACE
index 31c8103..61991e9 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -125,6 +125,7 @@
     "//tools/cpp:cc-toolchain-cortex-m4f",
     # Find a good way to select between these two M4F toolchains.
     #"//tools/cpp:cc-toolchain-cortex-m4f-k22",
+    "//tools/python:python_toolchain",
 )
 
 http_archive(