Minimal reproduction for rules_python #3495

rules_python 1.7.0 (PR #3242) introduced %interpreter_args% template placeholder in bootstrap scripts. External py_binary targets don't substitute this, causing:

File ".../cython_binary", line 39
    %interpreter_args%
    ^
SyntaxError: invalid syntax

Reproduction

# 1bb07d8: rules_python 1.6.3 - WORKS
git checkout 1bb07d8
bazel build @com_github_grpc_grpc//src/python/grpcio/grpc/_cython:cygrpc.pyx_cython_translation

# 3ca4b67: rules_python 1.7.0 - FAILS
git checkout 3ca4b67
bazel clean --expunge # if needed
bazel build @com_github_grpc_grpc//src/python/grpcio/grpc/_cython:cygrpc.pyx_cython_translation