Run a Python operator the way the engine runs it #8409

Task Summary

A Python operator is not called; it is handed to an interpreter that imports the generated module and drives it through the same open, process and close the engine uses. This step adds PyOpExecHarness, which writes that module, starts the driver, and reads back what the operator emitted.

The driver is the engine's side of the contract written out plainly. It is what makes the answer this side produces the engine's answer rather than an approximation of it.

Step 4 of 27 in #8325. It stands on its own: it touches no code from the export itself.