Use Python 3 for subprocesses

There is a matrix of what interpreter we use for under-the-hood and what we use for subprocesses. Presumably, when calling `./pants3`, you would expect to also run your tests and subprocesses with Py3.

This change is not only nice-to-have but also required because the native engine does not work when bootstrapped with Python 3 and running a Python 2 subprocess *if* that subprocess uses `native_engine.so`, as several of our tests do.

This may result in conflicts when trying to run Py2-only targets, e.g. python_antlr_library.py. If this is the case, we can resort to pantsbuild#7048 as an alternative solution.

pants3

@@ -4,5 +4,10 @@
 
 # This bootstrap script invokes Pants using a Python 3 interpreter.
 
+# Use Py3 under-the-hood
 export PANTS_USE_PYTHON3=true
+
+# Use Py3 for subprocesses
+export PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS='["CPython>=3.6,<4"]'
+
 ./pants "$@"