Keep the letters at the ends of an executor code block

[SashaMIT]

Summary

• The humaneval executor cleaned a code block with strip("python"). That removes the letters p, y, t, h, o, and n from both ends.
• return n came back as return . print(1) came back as rint(1). A python or python3 tag at the front is still removed. return n stays return n. print(1) stays print(1).

Test plan

• python -m unittest tests.test_humaneval_executor
• Red on tip: return vs return n, and rint(1) vs print(1). Green after: both lines stay intact.